install: support extra_system_packages config option

install: support extra_system_packages config option

On DEB systems, packages specified via the extra_packages option are installed
while forcing the same package version number as the version of the project
(i.e. Ceph) under test. So extra_packages can only be used to specify
additional project (Ceph) packages.

If we wanted to specify additional system (non-project, non-Ceph) packages to
install, we were out of luck. This commit implements an extra_system_packages
option for specifying extra non-project packages.

Fixes: http://tracker.ceph.com/issues/25026
Signed-off-by: Nathan Cutler <ncutler@suse.com>