Hiding device name field when unhandled

When OPENSTACK_HYPERVISOR_FEATURES['can_set_mount_point'] is set to false
we hide the device_name field in the launch_instance form

Change-Id: Id5eacbfc7fb230320e0d0bc8bcc839e285844f86
Closes-Bug: 1273298
Merge "Hiding device name field when unhandled"