Various bugfixes in handling of SSH execution

+ Bugfix in execute detection for Palo Alto (it was missing).
+ SSH-PTY will always be the default execution method vs. a "null"
  execute function
+ Bugfix in sending 'show run' for Nexus switches causing
  startup_command detection to crash for any IOS-like devices
+ Paging is now disabled on Juniper devices if force_cli is set