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.
plugin/ostree: Fix doc typos
Handle missing or empty ostree origin file.

If gi_wrapper.py doesn't find a ostree origin
file, skip updating. This can happen in cases where
ostree isn't installed, or gi_wrapper.py fails.
Merge pull request #976 from candlepin/alikins/ostree_config_bool

Fix saving ostree remote configs with gpg set.
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
Merge pull request #974 from cgwalters/doc-typos

plugin/ostree: Fix doc typos
Merge pull request #977 from candlepin/alikins/handle_no_origin

Handle missing or empty ostree origin file.
Handle Content.gpg="http://" as gpg-verify=false

Content.gpg is a url, not a boolean. So for cases
where it needs to be both, except the "http://"
value as equilivent to disabling gpg-verify.

Flip gpg-verify to enabled by default for ostree
content, and disabled with above url.

We will set gpg-verify=true always, unless the
Content.gpg attribute exists is = "http://".
Missing, empty, None, False,etc values for
Content.gpg do not flip or unset gpg-verify.

Add spot checks of gpg-verify to various unit
tests.