Don't ignore --port and --host CLI options for migrations after start

[dholdren]

Fixes issue where passing --port on command line for start subcommand was used when starting the server, but not for migrating.
This also adds the --host argument and fixes the same problem with it.

Reproduce:

Given a serverless.yml like this:

service: sls-dynamodb-test

provider:
  name: aws
  runtime: nodejs10.x

functions:
  hello:
    handler: handler.hello

plugins:
  - serverless-dynamodb-local

custom:
  dynamodb:
    stages:
      - dev

resources:
  Resources:
    DynamodbTable:
      Type: "AWS::DynamoDB::Table"
      Properties:
        TableName: test-table
        AttributeDefinitions:
          - AttributeName: testId
            AttributeType: S
        KeySchema:
          - AttributeName: testId
            KeyType: HASH
        ProvisionedThroughput:
          ReadCapacityUnits: 1
          WriteCapacityUnits: 1

If you run:
sls dynamodb start --port 8123 --inMemory --migrate

You will get the following error (the server is started, but migrating fails):

Dynamodb Local Started, Visit: http://localhost:8123/shell
NetworkingError: connect ECONNREFUSED 127.0.0.1:8000: DynamoDB - Error -

  Networking Error ---------------------------------------

  NetworkingError: connect ECONNREFUSED 127.0.0.1:8000
      at Object._errnoException (util.js:1022:11)
      at _exceptionWithHostPort (util.js:1044:20)
      at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1198:14)
  From previous event:
      at PluginManager.invoke (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/lib/classes/PluginManager.js:505:22)
      at getHooks.reduce.then (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/lib/classes/PluginManager.js:540:24)
  From previous event:
      at PluginManager.run (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/lib/classes/PluginManager.js:540:8)
      at variables.populateService.then (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/lib/Serverless.js:115:33)
      at runCallback (timers.js:794:20)
      at tryOnImmediate (timers.js:752:5)
      at processImmediate [as _immediateCallback] (timers.js:729:5)
  From previous event:
      at Serverless.run (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/lib/Serverless.js:102:74)
      at serverless.init.then (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/bin/serverless.js:72:30)
      at /Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/node_modules/graceful-fs/graceful-fs.js:111:16
      at /Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/node_modules/graceful-fs/graceful-fs.js:45:10
      at FSReqWrap.oncomplete (fs.js:135:15)
  From previous event:
      at initializeErrorReporter.then (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/bin/serverless.js:72:8)
      at runCallback (timers.js:794:20)
      at tryOnImmediate (timers.js:752:5)
      at processImmediate [as _immediateCallback] (timers.js:729:5)
  From previous event:
      at Object.<anonymous> (/Users/dean.holdren/.nvm/versions/node/v8.10.0/lib/node_modules/serverless/bin/serverless.js:57:4)
      at Module._compile (module.js:652:30)
      at Object.Module._extensions..js (module.js:663:10)
      at Module.load (module.js:565:32)
      at tryModuleLoad (module.js:505:12)
      at Function.Module._load (module.js:497:3)
      at Function.Module.runMain (module.js:693:10)
      at startup (bootstrap_node.js:188:16)
      at bootstrap_node.js:609:3

  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Issues:        forum.serverless.com

  Your Environment Information ---------------------------
     Operating System:          darwin
     Node Version:              8.10.0
     Framework Version:         1.54.0
     Plugin Version:            3.1.2
     SDK Version:               2.1.2
     Components Core Version:   1.1.1
     Components CLI Version:    1.2.3

Changes:

modified the get port() and get host() methods to allow command line arguments to override serverless.yml configuration and defaults.