Fixes prompt doubling issue that impacted missing ios enable password prompts. (#33794)

* Fixes a prompt doubling issue that impacted missing ios enable password prompts.

Due to get_prompt sending a '\n' the prompts became doubled and out-of-sync with what
was expected.  This caused the enable command prompts to be missed.
Also added verification that on_become succeeded to reach enable prompt.

* Moved prompt doubling comment per shippable

lib/ansible/plugins/terminal/__init__.py

@@ -69,7 +69,10 @@ class TerminalBase(with_metaclass(ABCMeta, object)):
 
         :returns: A byte string of the prompt
         """
-        self._exec_cli_command(b'\n')
+        # do not send '\n' here, exec_cli_command sends '\r' already,
+        # doing so causes double prompts.
+        self._exec_cli_command(b'')
+
         return self._connection._matched_prompt
 
     def on_open_shell(self):

lib/ansible/plugins/terminal/ios.py

@@ -65,8 +65,12 @@ class TerminalModule(TerminalBase):
 
         try:
             self._exec_cli_command(to_bytes(json.dumps(cmd), errors='surrogate_or_strict'))
+            prompt = self._get_prompt()
+            if not prompt.endswith(b'#'):
+                raise AnsibleConnectionFailure('failed to elevate privilege to enable mode still at prompt [%s]' % prompt)
         except AnsibleConnectionFailure:
-            raise AnsibleConnectionFailure('unable to elevate privilege to enable mode')
+            prompt = self._get_prompt()
+            raise AnsibleConnectionFailure('unable to elevate privilege to enable mode, at prompt [%s]' % prompt)
 
     def on_unbecome(self):
         prompt = self._get_prompt()