Moved out common testcases from test_type_vxlan.py

This patch-set will refactor the test_type_vxlan.py and test_type_gre.py.

We need to add testcases for TunnelRpcCallbackMixin() so better we
should move common test cases from test_type_vxlan.py to test_type_tunnel.py and
add testcases for TunnelRpcCallbackMixin() into it.

Partial-Bug: #1381071
Change-Id: I81f8930cf665ccd89c03f0bd2199ce5d1d9dff65
Merge "Moved out common testcases from test_type_vxlan.py"