googlesource.com runs close to master and the latest gerrit RC is
closest to that. Furthermore, this allows the tests to use the latest
REST api endpoints (including create account).

Fix the sorting of gerrit versions to extend the major revsion numbers
before the RC numbers. This makes 2.7-rc4 comparable to 2.7.1.

The gerrit init command now requires download-commands plugin to be
installed in order for the revisions fetch field to be populated (i.e
to show the download schemes).

NOTRY=true

Review URL: https://codereview.chromium.org/69373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@238631 0039d316-1c4b-4281-b951-d872f2087c98