Mask environment configuration with no_log

[dekimsey]

I ran into this when I was configuring the mongo exporter, it requires me to set credentials via environment variables.

Environment files are frequently used to inject sensitive values to daemons; this change enables the no_log flag on this task.

Alternatively, we could have two tasks that do the same thing and require users to opt out/in perhaps something like this:

prometheus_my_exporter_env_vars:
   foo: 'not a secret, i like my diffs'
prometheus_my_exporter_env_insensitive: true

[dekimsey]

Looks like some other exporters use configuration file based bits that pass credentials around. So I'm thinking this may need to be scoped out a bit more.