Create new keyword that performs sanity test procedure

Currently sanity test is defined in the form of a test case and robot framework doesn't support an easy way to call a certain test case from another test case.

Sanity test keyword does the following

 - authentication, DHCP and pings on all the ONUs passed in the test

By creating a new keyword we can call "Sanity Test" in any other test easily.  This is required especially in scenarios where a recheck of sanity test is required after certain operations