pq-rpm: implement '--import-files' command line option

pq-rpm: implement '--import-files' command line option

For defining the packaging file(s) that will be imported into the
development/patch-queue branch.

By default, the local gbp conf files are imported in order to try to
ensure that gbp sees the same settings on the development/pq branch as
on the packaging branch.

NOTE: This option does not affect the patch files that are imported. The
files defined with this option will appear as new files in one
monolithic commit in the development/patch-queue branch.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>