Enable SNAT by default in L3 agents

If ext-gw-mode extension isn't supported by plugin, it won't
return enable_snat param in router info. Agent will currently
default to enable_snat = false, which changes from expected
default behaviour prior to ext-gw-mode introduction.

This patch changes L3 agent to enable SNAT by default if plugin
doesn't support ext-gw-mode extension.

Change-Id: I35e8f8c20392bff9ac2f875f2c9a1038ab06ad7b
Closes-Bug: #1212868
Merge "Enable SNAT by default in L3 agents"