Global use of table ids from NwConstants.java

Global use of table ids from NwConstants.java

This review includes following things:

1> After merging of https://git.opendaylight.org/gerrit/#/c/42940/, build
of Netvirt started failing. Because ElanStatisticsImpl.java was using one
constant from IfmConstants.java, which was removed in mentioned commit.

2> Removed few table ids from FibConstants.java, NATConstants.java and
VpnSubnetRouteHandler.java then using directly from NwConstants.java.

Change-Id: Ie6d1663113d7f8dcb1965954d51776d98fd73ddb
Signed-off-by: Suraj Ranjan <suraj.ranjan@ericsson.com>