gbp-pq: readiness to configure the pq branch name

gbp-pq: readiness to configure the pq branch name

All other gbp branches have configurable names. This commit adds the
readiness for user to configure/change the name of the patch-queue
branches, as well.

Patch-queue is defined in options as a format string, where '%(branch)s'
refers to the debian/packaging branch. If the pq-branch format string
does not contain '%(branch)s', there is only one patch-queue branch and
the debian/packaging branch is used as its base branch. That is, e.g. a
'gbp-pq switch' operation from the patch-queue branch always switches to
the debian/packaging branch.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Olev Kartau <olev.kartau@intel.com>
Conflicts:
	gbp/scripts/common/pq.py