[telemetry] Add credentials login url to be overridden

When working with staging systems, you need to be able to provide an
alternative login url.

R=tonyg,dtu

Review URL: https://codereview.chromium.org/28623003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229726 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229726