rbt login/logout commands crash if the server url is set in the SCM config
The rbt login and logout commands do not initialize the scm tool like other

commands do. This means that if the server url is set in the scm (through 

.gitconfig for example), rather than the command line then login and logout

will crash with an AttributeError.

Tests pass except for the MercurialClient (+svn) tests, where I'm seeing this issue with hgsubversion: https://bitbucket.org/durin42/hgsubversion/issues/449/extension-breaks-mercurial-36-rc2
Additionally, it seems the MercurialClient (+svn) test failure will leave the suite in a bad state because all subsequent tests will fail with an OSError, but when run in isolation they pass just fine. The travis-ci build hit the same issue: https://travis-ci.org/reviewboard/rbtools/jobs/94449505