trust: use context.principal only when it is defined

In server-like context we use LDAPI connection with auto-binding to LDAP
object based on the UID of the process connecting to LDAPI UNIX domain
socket. This means context.principal is not set and we cannot use it.

Make sure to reject requests unless we are operating as a Directory
Manager in such cases.

Fixes: https://pagure.io/freeipa/issue/9583

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Thomas Woerner <twoerner@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Rafael Guterres Jeffman <rjeffman@redhat.com>

ipaserver/plugins/trust.py

@@ -1872,7 +1872,8 @@ class trust_enable_agent(Command):
 
         # the user must have the Replication Administrators privilege
         privilege = u'Replication Administrators'
-        if not principal_has_privilege(self.api, context.principal, privilege):
+        op_account = getattr(context, 'principal', None)
+        if not principal_has_privilege(self.api, op_account, privilege):
             raise errors.ACIError(
                 info=_("not allowed to remotely add agent"))