neutron router-gateway-set failed for non-admin users

Fixed the neutron router-gateway-set command regression:
"neutron router-gateway-set <router> <public-net>"
is failing for non-admin users.

Changed the behavior to pass the enable_snat attribute to
the server only when the --disable-snat option is specified.

bug 1209027

Change-Id: Iec55a17ed7966ee15bf05982d98540fe6884fa20
Merge "neutron router-gateway-set failed for non-admin users"