1112282: Dont build ostree plugin subpackage < 7

Don't build or install the ostree-plugin subpackage
except on systems that could use it. It requires
pygobject-3 which is not available on RHEL6.

Add a INSTALL_OSTREE_PLUGIN makefile variable. Default
to true. Override it from spec for older releases.

Some Makefile reorganizing to support that.
This should be ```%global has_ostree %{use_systemd}```