Avoid full elan-interface read for gre/vxlan ext-network

Avoid full elan-interface read for gre/vxlan ext-network

Description:
To handle flat/vlan provider type use-cases, when the subnet is created,
a NAT listener used to query the full elan-interface and try to identify
specific elan-interfaces associated with physical interfaces.

THis call is not required for every subnet creation and also for
gre/vxlan provider type networks.

Check been added to prevent unnecessary getAllElanInterface call to
prevent bulk read.

Change-Id: Id91797c047e0a45380d1cd0cddf963228fc645ac
Signed-off-by: Chetan Arakere Gowdru <chetan.arakere@altencalsoftlabs.com>