Make --address, --password-file settable from env

[peb-adr]

Might be useful if we decide to put osmanage into backend image.
This way we can also set env vars

• OSMANAGE_BACKEND_ADDRESS
• OSMANAGE_BACKEND_PASSWORD_FILE

to container-appropriate values such that manage commands talking to backend will just work.

[peb-adr]

Discussed with @4echow that the code duplication introduced by me now could be reduced. Also tests should probably be written..

However we should first be sure we want this before making it very pretty.