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.
make has_ostree use macro value NOT hardcoded value.

has_ostree was being set to use_systemd which caused it to always
be set and considered TRUE. We really wanted it to be the *value*
of use_systemd. In order to do that you need to use the percent
sign to make it a macro so it gets expanding during build time.
Merge pull request #973 from candlepin/alikins/1112282_cond_ostree_rpm

1112282: Dont build ostree plugin subpackage < 7