Fixes#37208
If check_mode is enabled instead of committing th config need to
discard all the chnages to cnadidate db
In case of cli to discard changes issue `rollback 0` command
and for netconf execute `discard-changes` rpc call
* Add connection=netconf in individual roles for modules that run using netconf connection
plugin
* Add connection=network_cli for junos_netconf and junos_command
at applicable places
* Add junos_banner declartive module
* junos_banner implementation
* Integration test for junos_banner
* Integration test for net_banner (junos)
* Minor fixes
* Minor doc change
