Push the username/password is None logic down into the interactive prompt method

This will make it so rbt post is bug compatible with post-review
when I specify --username on the command line, so it doesn't
always ask me for a username if I specified one via --username

This resolves Issue 3114

Sponsored-by: Zonar Systems, Inc
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Use httplib constants instead of hardcoded values

Sponsored-by: Zonar Systems, Inc
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Don't bomb out if a password wasn't provided

The base class can return None if the password attribute
is None

Sponsored-by: Zonar Systems, Inc
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Initialize the superclass for ReviewBoardHTTPPasswordMgr in the constructor

Found-by: pylint

Sponsored-by: Zonar Systems, Inc
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Merge branch 'master' of github.com:yaneurabeya/rbtools