ipaclient: implement thin client

Dynamically create plugin package for the remote server with modules and
commands based on the API schema when client API is finalizes. For in-tree
API instances, use ipalib.plugins directly.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/plugable.py

@@ -524,7 +524,7 @@ class API(ReadOnly):
             )
 
         self.log.debug("importing all plugin modules in %s...", package_name)
-        modules = find_modules_in_dir(package_dir)
+        modules = getattr(package, 'modules', find_modules_in_dir(package_dir))
         modules = ['.'.join((package_name, name)) for name in modules]
 
         for name in modules: