Use common get_instance call in API plugins part 3

Cleanup v2.1 API to use common get_instance call which
automatically does exception handling. Although most of
the v2.1 API was cleaned up a while back, some recent
ports of v2 API code has resulted in some cases creeping
backing in.

Change-Id: I8271a2805fe9fdfae40df43183a5251fddbe4c13
Adds new RT unit tests for _sync_compute_node

Adds new unit tests for the resource tracker's _sync_compute_node()
method. These new unit tests should make the objectification of the
resource tracker easier to do, since the old unit tests in
test_resource_tracker.py are brittle and hard to change.

Change-Id: I3e8d179d8c3856084fee595c8b0396a8a5db8aa4
Blueprint: make-resource-tracker-use-objects
Merge "Use common get_instance call in API plugins part 3"
Merge "Adds new RT unit tests for _sync_compute_node"