Remove order dependence from network_basic_ops

There is no guarantee on runtime order for tests, however
test_network_basic_ops had a separate test case for each step in
the scenario being run. This commit just changes the test structure
to individually run the steps of the test scenario inside a single
test method.

Change-Id: I729959b7a11a4c4413c3a7c262318725c56871c1
Add @services decorator

This commit adds a new decorator used for specifying service tags for
a test. It just wraps the attr decorator but before adding the attr it
checks to make sure a valid service name was used.

Part of bp add-service-tags

Change-Id: I0cdf3f5c703a5e4573b8e9b26fb8ede9b6d77690