Use L3 api from vpn ipsec driver via service plugin

VPNaaS and ML2 plugin won't work, because ML2 plugin
supports service version of L3.
In this commit, we modify ipsec driver to use L3 plugin.
This is also backward compatible change, because if L2 plugin
supports L3 get_service_plugin API returns L2 plugin.

Fixes bug 1238846

Change-Id: I36e541bb2e3e1df2e01f73a74f3e9005af6c38b7
Note: Exsiting unit test covers this change
Merge "Use L3 api from vpn ipsec driver via service plugin"