Correct inconsistent enable_snat management

Correct inconsistent enable_snat management

Neutron resets enable_snat attribute when external_gateway_info is
cleared but not when external_gateway_info is only updated which
implies the following sets of actions have different behaviors:

  neutron router-gateway-set router1 pub1 --disable-snat
  neutron router-gateway-set router1 pub2

enable_snat is False after the last command

  neutron router-gateway-set router1 pub1 --disable-snat
  neutron router-gateway-clear router1
  neutron router-gateway-set router1 pub2

enable_snat is True after the 2nd command resets the gateway AND
enable_snat.

This change proposes to always reset the attribute enable_snat when
enable_snat is not provided in external_gateway_info on POST/PUT for
consistency.

APIImpact
Change-Id: Ibab289936c55b1cf9614b44a4f18f54c959ee9e8
Closes-Bug: #1384146