1967780: improve placeholders in help text

When describing the format of the argument for '--proxy', use different
(and generic) placeholders to describe the format needed; do not use the
same names of configuration keys to avoid potential confusion.

src/subscription_manager/cli_command/cli.py

@@ -165,7 +165,7 @@ def _add_url_options(self):
     def _add_proxy_options(self):
         """ Add proxy options that apply to sub-commands that require network connections. """
         self.parser.add_argument("--proxy", dest="proxy_url",
-                               default=None, help=_("proxy URL in the form of proxy_hostname:proxy_port"))
+                               default=None, help=_("proxy URL in the form of hostname:port"))
         self.parser.add_argument("--proxyuser", dest="proxy_user",
                                 default=None, help=_("user for HTTP proxy with basic authentication"))
         self.parser.add_argument("--proxypassword", dest="proxy_password",