Add missing tests for compute_node_* methods

There was some missing tests in test_db_api for
compute_node_* methods.

Add tests to ensure that all works.

Also: old compute_node_* methods tests refactoring,
test_compute_node_stat_unchanged() rewrited
(sqlalchemy_api dependence removed).

blueprint db-api-tests

Change-Id: I0204eba442e2c6bbc06a238bbdff1eb236a713db
Merge "Add missing tests for compute_node_* methods"