Working around #11 until the next Netmiko release

Until there will be a more permanent solution in Netmiko, in order to
have this driver usable with version 4.x, we need to "help" Netmiko when
it fails to detect the prompt. Strictly speaking, as discussed under
ktbyers/netmiko#2866 Netmiko works as
designed, the ``base_prompt`` attribute is what we'd expect to be, but
when OpenGear doesn't set anything on the prompt other than ``$`` or ``#``
depending on the user elevation, then Netmiko fails to properly read
(more specifically, won't know when to stop).

I also took this chance to change the behaviour in ``open()`` to only
invoke ``enable()`` when the user is not root -- this saves one
unnecessary operation when we use the root user.

napalm_opengear/opengear.py

@@ -58,7 +58,14 @@ def open(self):
                                      global_delay_factor=2,
                                      **self.netmiko_optional_args
         )
-        self.device.enable()
+        if not self.device.base_prompt:
+            # If Netmiko fails to detect the prompt due to OpenGear not setting
+            # anything before the pound sign, we set the base_prompt attribute
+            # so Netmiko is able to read (either # when the username is root, or
+            # $ for anything else)
+            self.base_prompt = "#" if self.username == "root" else "$"
+        if self.username != "root":
+            self.device.enable()
 
     def close(self):
         self._netmiko_close()