Move flags in powervm/driver into a powervm option group

This change moves powervm specific flags into a new "powervm"
options group.

DocImpact: the following flags have been moved to this group (I
have dropped the powervm_ prefix from the flags at the same time):
    powervm_mgr_type
    powervm_mgr
    powervm_mgr_user
    powervm_mgr_passwd
    powervm_img_remote_path
    powervm_img_local_path

Implements blueprint: powervm-opts-group

Change-Id: Ie896b00a2813c219661f662166a7987d24de4400