Run SNAT specific test cases only with ext-gw-mode extension

In network api's test_routers, there are test cases which involves
enable_snat attribute. We should not be running these test cases if
the ext-gw-mode extension is not supported by network plugin.
Adding decorator to corresponding methods in test_routers.

Change-Id: I71fd96ce6b30ba7307c92733c363eb996ee6c1db
Closes-Bug: #1273360
Merge "Run SNAT specific test cases only with ext-gw-mode extension"