Only mount selinux sysfs in nova_libvirt container

Only mount selinux sysfs in nova_libvirt container

https://review.openstack.org/500952 initially just did this. Then we assumed
every container should have the selinux sysfs.
This causes issues with the sshd container used for live-migration.

The advice from the selinux experts is that it should not be enabled within
containers, so reverting back to the original fix that enables it only in the
nova-libvirt container.

Closes-bug: 1729405
Change-Id: I80bf38d7d64ab99510574af5c57423fde9b84eca
(cherry picked from commit 7c8127cf96a281dd5cee96e1a68bc0508b9ba4e7)