Move set_admin_password tests from test_compute.py to api/mgr modules

To prepare for converting the set_admin_password compute API flow to use
objects consistently, i.e. no direct database updates, use dot notation
to access instance object attributes, etc, the unit tests need to be
refactored and moved from test_compute.py into test_compute_api.py and
test_compute_mgr.py to properly unit test the compute api and manager
changes, respectively.

As part of the move, the tests are re-written with mock.

Part of blueprint compute-manager-objects-juno

Change-Id: I5815f7dec3105440f1b211d7bac98888fde7a576
Merge "Move set_admin_password tests from test_compute.py to api/mgr modules"