Improve help strings

Follow oslo.config conventions for consistency of help strings:
* Use sentence style capitalization.
* End entry with a "."
Fix capitalization of AMQP.

Change-Id: I0a6b170a2a7cfa539e06781f4495c4353d22e4d0

oslo/messaging/_drivers/amqp.py

@@ -39,10 +39,10 @@ amqp_opts = [
                 default=False,
                 deprecated_name='rabbit_durable_queues',
                 deprecated_group='DEFAULT',
-                help='Use durable queues in amqp.'),
+                help='Use durable queues in AMQP.'),
     cfg.BoolOpt('amqp_auto_delete',
                 default=False,
-                help='Auto-delete queues in amqp.'),
+                help='Auto-delete queues in AMQP.'),
 
     # FIXME(markmc): this was toplevel in openstack.common.rpc
     cfg.IntOpt('rpc_conn_pool_size',

oslo/messaging/_drivers/impl_rabbit.py

@@ -78,7 +78,7 @@ rabbit_opts = [
                secret=True),
     cfg.StrOpt('rabbit_login_method',
                default='AMQPLAIN',
-               help='the RabbitMQ login method'),
+               help='The RabbitMQ login method.'),
     cfg.StrOpt('rabbit_virtual_host',
                default='/',
                help='The RabbitMQ virtual host.'),