install: prefer 'branch' over 'sha1'

The upgrade tasks specify 'branch' in the job file, but the
schedule_suite.sh script sets a sha1 in the overrides.  Make
the upgrade tests actually test an upgrade by preferring branch
over sha1 when both are specified.

This is fragile, but ought to do the trick for now!

Signed-off-by: Sage Weil <sage@inktank.com>