Fix handling of the --username and --password options to match post-review.

The way that the --username and --password options were handled were a little
annoying, and worked differently between rbt and post-review. This change
pushes the username/password is None logic down into the interactive prompt
method, and adds some checks to avoid prompting if these are specified via the
command line.

Testing done:
Posted reviews with rbt when specifying --username only and saw only the
password prompt.

Reviewed at https://reviews.reviewboard.org/r/4785/
Fixes bug 3114.