cf_exporter: replace CLI args with ENV args

[aegershman]

In order to prevent credentials passed to cf_exporter being visible in the VM's process table, this swaps out passing CLI args to cf_exporter in favor of leveraging exporting the equivalent env args

This would be the first of a few commits/PRs to update other exporters, e.g. firehose-exporter, bosh-exporter, etc.

Open questions/comments:

• Should only credentials be swapped out to leverage env vars? E.g., allow for non-credential args to be passed via CLI so they're still visible in ps aux? Reason for not doing this is to maintain consistency.
• Is current spacing acceptable? Or should it have no newlines inbetween?

[frodenas]

@aegershman I don't think we need to switch all flags to envvars, just the ones that are sensitive, like client id and secret.

[aegershman]

fixed 👍 in general I tend to leave username/client-id in plaintext, though would you all prefer they be moved to envvars as well? It's no problem either way.

[frodenas]

Thanks @aegershman