RHEL-7206: Small change of message printed by dnf plugin

* When system is not registered, then we print hint how to
  register the system. We mention only "subscription-manager",
  but there is no information about "rhc", when "rhc" is
  installed.
* This patch adds "rhc" to this message.
* When "rhc" is not installed, then original message is
  displayed.

src/plugins/dnf/subscription_manager.py

@@ -14,6 +14,7 @@
 
 import os
 import logging
+import shutil
 
 from subscription_manager import injection as inj
 from subscription_manager.action_client import ProfileActionClient
@@ -48,6 +49,12 @@
 """
 )
 
+not_registered_warning_rhc = _(
+    """
+This system is not registered with an entitlement server. You can use "rhc" or "subscription-manager" to register.
+"""
+)
+
 no_subs_warning = _(
     """
 This system is registered with an entitlement server, but is not receiving updates. You can use \
@@ -182,7 +189,10 @@ def _warn_or_give_usage_message():
             ent_dir = inj.require(inj.ENT_DIR)
             # Don't warn people to register if we see entitlements, but no identity:
             if not identity.is_valid() and len(ent_dir.list_valid()) == 0:
-                msg = not_registered_warning
+                if shutil.which("rhc") is not None:
+                    msg = not_registered_warning_rhc
+                else:
+                    msg = not_registered_warning
             elif len(ent_dir.list_valid()) == 0 and not is_simple_content_access(identity=identity):
                 msg = no_subs_warning
         finally: