Support specification of multiple PBS client hosts in PTL
Currently PTL supports specification of a single host for "client" parameter, i.e. name of the host on which the PBS client commands are to be run from. This needs to be enhanced to support mention of multiple clients; which might be a use case for various types of testing ex: Performance test where at a time multiple qsubs are triggered from different hosts or the PBS server is stated simultaneously for job information from different hosts etc.
This requirement was noticed during work on @requirements decorator which supports specifying multiple clients necessity for a given test case. https://pbspro.atlassian.net/wiki/spaces/PD/pages/458883073/PP-1281+New+decorator+in+PTL+using+which+user+can+provide+cluster+information+required+for+a+test.