This CL introduces new top level command for managing cached auth tokens:
  $ depot-tools-auth login codereview.chromium.org
  $ depot-tools-auth info codereview.chromium.org
  $ depot-tools-auth logout codereview.chromium.org

All scripts that use rietveld.Rietveld internally should be able to use cached
credentials created by 'depot-tools-auth' subcommand. Also 'depot-tools-auth'
is the only way to run login flow. If some scripts stumbles over expired or
revoked token, it dies with the error, asking user to run
'depot-tools-auth login <hostname>'.

Password login is still default. OAuth2 can be enabled by passing --oauth2 to
all scripts.

R=maruel@chromium.org
BUG=356813

Review URL: https://codereview.chromium.org/1074673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294764 0039d316-1c4b-4281-b951-d872f2087c98