Enable attribute-based policy on router:external_gateway_info

Bug 1186081

This patch also removes a check on is_write in policy.py since
the code block where that check is placed assumed already that
is_write == True

Change-Id: I21c54f63e1948675f67afb088c262dc5316c230d
Merge "Enable attribute-based policy on router:external_gateway_info"