added option to choose ip alloc from start or end, modified gw_ip based on ip alloc option, added check in network_update to reject modified allocation pool, dnsservers and gateway_ip

Conflicts:
	src/config/api-server/vnc_addr_mgmt.py
	src/config/api-server/vnc_cfg_ifmap.py
	src/schema/vnc_cfg.xsd

Change-Id: I35a52e2093d14f8e4815de7d2de9159a5a4afffc
Merge "added option to choose ip alloc from start or end, modified gw_ip based on ip alloc option, added check in network_update to reject modified allocation pool, dnsservers and gateway_ip"