Clarify zuul-cloner message about missing branch

Inform correctly about the missing branch in case that ZUUL_BRANCH is
set, but options --branch is empty.

Before:
INFO:zuul.Cloner:upstream repo is missing branch None
After:
INFO:zuul.Cloner:upstream repo is missing branch stable

Also do not print out anything when ZUUL_BRANCH nor --branch is given.

Change-Id: I74b3e72b3066f03bcf3a10af3657bdf494d3cc6b
1 file changed