Fix authentication bug when booting an server in V3

Currently when booting a server with V3, novaclient sends an empty
os_password to image_cs. This will cause 401(Unauthorized: Invalid
user/password) when trying to find image. This is is a result of
changes nova's V3 API: nova is no longer used as a proxy for the
image service. So novaclient uses two Client instances: one for
nova, the other for image service. This patch checks os_password
before creating the image Client and assigns it if it's empty.

Change-Id: Ic54cef93e9b823fb98b1edd78776c9a1fc06ba46
Closes-Bug: #1277425
Merge "Fix authentication bug when booting an server in V3"