Add terminal_initial_newline to network_cli module

`terminal_initial_newline`, when set to true sends a single newline
character to the remote device immediately after initially connecting.

This allows network_cli to work with some types of equipment which
fail to send any data when SSH connectivity is initially established,
such as serial-port multiplexers.

changelogs/fragments/terminal_initial_newline.yaml

@@ -0,0 +1,2 @@
+minor_changes:
+  - network_cli - add `terminal_initial_newline` parameter as an option for terminal plugins.

plugins/connection/network_cli.py

@@ -198,6 +198,16 @@
       response, for example I('re.I').
     vars:
     - name: ansible_terminal_stderr_re
+  terminal_initial_newline:
+    type: boolean
+    description:
+    - This boolean flag, that when set to I(True) will send newline on initial
+      connection establishment to the remote device.
+    - This can be useful for equipment which does not send an initial header until
+      it receives some input, like Serial-to-SSH multiplexer hardware.
+    default: false
+    vars:
+    - name: ansible_terminal_initial_newline
   terminal_initial_prompt:
     type: list
     elements: string
@@ -656,6 +666,10 @@ def _connect(self):
                 "loaded terminal plugin for network_os %s" % self._network_os,
             )
 
+            terminal_initial_newline = (
+                    self.get_option("terminal_initial_newline") or self._terminal.terminal_initial_newline
+            )
+
             terminal_initial_prompt = (
                 self.get_option("terminal_initial_prompt") or self._terminal.terminal_initial_prompt
             )
@@ -668,6 +682,13 @@ def _connect(self):
             )
             check_all = self.get_option("terminal_initial_prompt_checkall") or False
 
+            if terminal_initial_newline:
+                self.send(
+                    command=b"",
+                    sendonly=True,
+                    newline=True
+                )
+
             self.receive(
                 prompts=terminal_initial_prompt,
                 answer=terminal_initial_answer,

plugins/plugin_utils/terminal_base.py

@@ -40,6 +40,9 @@ class TerminalBase(TerminalBaseBase):
         re.compile(rb"\x1b\[m"),  # ANSI reset code
     ]
 
+    #: Send newline after initial session connection
+    terminal_initial_newline = False
+
     #: terminal initial prompt
     terminal_initial_prompt = None