authentication.  Also allow apply_issue.py to specify a username if needed.

BUG=43563
TEST=Make sure that sending try jobs directly from rietveld works.

Review URL: https://chromiumcodereview.appspot.com/10704111

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@145600 0039d316-1c4b-4281-b951-d872f2087c98