Allow to define enable_snat default value

Allow to define enable_snat default value

Currently neutron resets enable_snat attribute to True when enable_snat
is not provided in router external_gateway_info. But in some deployments
(private/enterprise clouds) such behavior is not the expected default
one as snat/nat/floating-ips is not used (at least by default).

This change defines the option enable_snat_by_default which allows
deployers to set enable_snat default value when neutron resets it. The
option default value is True for backward compatibility.

DocImpact
APIImpact
Closes-Bug: #1388858
Change-Id: I455a552230ec89fe907a087c1de8c8144b5d086e