Adding option to take user credential

Provision script provision_forwarding_mode.py does not have option to provide user credential.
It only tries to access vnc api using default value. After multi tenancy is enable by default,
all the user need this option to provide user creadential for the forwarding script. Enhancing
the the script to take user credential.
Change-Id: If24fc32c6308fae162464f87f088c34b0c75ac93
Merge "Adding option to take user credential" into R2.0