Heat test modified to support neutron subnets.

Added SubnetId attribute to the stack template,
implemented method for retrieving subnet-id if Neutron enabled,
create_stack, update_stack methods generalized to avoid
code duplication,

Change-Id: Ie58a9ad08fe620965405e30283170874ed52e725
Closes-Bug: #1262163
Merge "Heat test modified to support neutron subnets."