fence_ibm_powervs: add private endpoint and token file support (#597)

• Enhance fence_ibm_powervs.py

New features:

• support PER-enabled Power Virtual Server workspaces with network access to private endpoints
• support security token to load from file

Fixes:

• support unknown states for virtual system instances
• new defaults for variables power_timeout = "120" power_wait = "15" stonith_status_sleep = "10"
• changed longdesc