Enable Core Resource Creation with Fixed ID

Neutron API currently blocks creation of resources with a fixed Id.
UUID is generated by the DB layer for every new resource creation.
However it is already enabled in the Neutron DB layer to specify a fixed
Id to the resource.

Few plugins (for example VMware) already uses this feature in the DB layer
to create resources with specific Id's.
There are several future use cases to use this, for example multi site
OpenStack and containers networking and possibly others in the area of
cloud orchestration.

Enabling Neutron API level to support resource creation with a fixed Id.
At this stage the changed resources are just core resources including network, subnet
port and subnet pools.

Enabling this can be done only for "PUT" REST calls for compliance, but this is
open for discussion.