Be clear about hypevisors.search used in a few CLIs

The following CLIs are doing a pattern match on the hypervisor
hostname when getting servers:

1. nova host-evacuate
2. nova host-evacuate-live (terrible name)
3. nova host-servers-migrate
4. nova host-meta (terrible name)

The fact that the hypervisor host(s) are looked up using
a pattern match is not clear in the help string for the host
argument.

This makes the help more clear and adds a warning about being
specific if you only want to target a specific host for these
operations.

In a later change we may modify the behavior in these CLIs which
allow you to find hypervisors via pattern matching, but for now
we should at least update the help text so we can backport it.

Change-Id: Icec03326bb4d9f898c04e10199038167ce5e3cce
Partial-Bug: #1667794

novaclient/v2/shell.py

@@ -4750,7 +4750,10 @@ def _server_evacuate(cs, server, args):
                                                "error_message": error_message})
 
 
-@utils.arg('host', metavar='<host>', help='Name of host.')
+@utils.arg('host', metavar='<host>',
+           help='The hypervisor hostname (or pattern) to search for. '
+                'WARNING: Use a fully qualified domain name if you only '
+                'want to evacuate from a specific host.')
 @utils.arg(
     '--target_host',
     metavar='<target_host>',
@@ -4813,7 +4816,10 @@ def _server_live_migrate(cs, server, args):
                                     error_message)
 
 
-@utils.arg('host', metavar='<host>', help='Name of host.')
+@utils.arg('host', metavar='<host>',
+           help='The hypervisor hostname (or pattern) to search for. '
+                'WARNING: Use a fully qualified domain name if you only '
+                'want to live migrate from a specific host.')
 @utils.arg(
     '--target-host',
     metavar='<target_host>',
@@ -4886,7 +4892,10 @@ def _server_migrate(cs, server):
                                        "error_message": error_message})
 
 
-@utils.arg('host', metavar='<host>', help='Name of host.')
+@utils.arg('host', metavar='<host>',
+           help='The hypervisor hostname (or pattern) to search for. '
+                'WARNING: Use a fully qualified domain name if you only '
+                'want to cold migrate from a specific host.')
 def do_host_servers_migrate(cs, args):
     """Cold migrate all instances off the specified host to other available
     hosts.
@@ -4963,10 +4972,10 @@ def do_list_extensions(cs, _args):
     utils.print_list(extensions, fields)
 
 
-@utils.arg(
-    'host',
-    metavar='<host>',
-    help=_('Name of host.'))
+@utils.arg('host', metavar='<host>',
+           help='The hypervisor hostname (or pattern) to search for. '
+                'WARNING: Use a fully qualified domain name if you only '
+                'want to update metadata for servers on a specific host.')
 @utils.arg(
     'action',
     metavar='<action>',