Reuse self.api when executing ca_enabled_check
The ca_enabled_check function is a wrapper around
api.Command.ca_is_enabled. When using remote_api (e.g. during
installer), ca_enabled_check invokes the *global* api instead of the
remote_api.
Update ca_enabled_check to explicitly receive an api object from the
caller and invoke Command.ca_is_enabled through it.
Part of: https://fedorahosted.org/freeipa/ticket/2614
Reviewed-By: Jan Cholasta <jcholast@redhat.com>