move MSCSTemplate classes to ipalib
As we expand the integration tests for external CA functionality, it
is helpful (and avoids duplication) to use the MSCSTemplate*
classes. These currently live in ipaserver.install.cainstance, but
ipatests is no longer permitted to import from ipaserver (see commit
81714976e5e13131654c78eb734746a20237c933). So move these classes to
ipalib.
Part of: https://pagure.io/freeipa/issue/7548
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>