Cells: Update set_admin_password for objects

Cells code was calling the compute/api set_admin_password method with an
old-world style instance and failing on the task_state attribute access.
This adds set_admin_password to the list of methods handled via the
cells RPC redirect method which is objects compatible.

Change-Id: I12e212260ed62dc9380202de31fda928a9498dcb
Closes-Bug: #1335315
Merge "Cells: Update set_admin_password for objects"