Refactor admin_action plugin and test cases

use common.get_instance()instead of self.compute_api.get()
add expected_attrs=None when setup mock in test case.

this patches is aimed to share v2.1 patches and v2 test cases.
a test cases porting patch will be followed.

Partially implements blueprint v2-on-v3-api
Change-Id: Iea7b0c0c2c920108ef055681319d8a2d996e14b3
Merge "Refactor admin_action plugin and test cases"