install.upgrade: fix overrides of sha1|tag|branch

If the upgrade task config has a branch: (or tag or sha1), do not apply the
sha1|branch|tag overrides keys.  This fixes the breakage from
280f783c2e8dda0df6afb4de0b115aad1614fbdc which made

overrides:
 install:
   ceph:
     sha1: f88866fe9c218e03b3f89ff21e2b7b655aa74294 tasks:
...
- install.upgrade:
   all:
     branch: dumpling

...use the sha1 from the overrides instead of the explicitly specified
branch.  The intention was to only use the overrides when the version was not
specified (whether it was sha1 or branch or tag).

At some point we should probably make the same change for install function in
install.py, but let's fix this first to get the upgrade tests working.

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