Add flag DEVSTACK_GATE_NEUTRON and enable heat grenade plugin
Add flag DEVSTACK_GATE_NEUTRON and enable heat grenade plugin

devstack-gate expects the DEVSTACK_GATE_NEUTRON flag to be
set in order to create the subnet with the right gateway.

The gate job is also modified to work with the newest external
plugin code in Grenade -> https://review.openstack.org/#/c/185050/

Change-Id: I0f02b00dff6e2c3bf9100292243f2f6e20937d27

Tesora/tesora-project-config,dongwenjuan/project-config,anbangr/osci-project-config,Tesora/tesora-project-config,anbangr/osci-project-config,coolsvap/project-config,coolsvap/project-config,noorul/os-project-config,noorul/os-project-config,openstack-infra/project-config,dongwenjuan/project-config,openstack-infra/project-config