Commit 00a74d1137 change the OVS
package compile from source, this makes some neutron staduim
projects func gate has no permission accessing the "ovs" dir.
This patch adds a BUILD_OVS_FROM_SOURCE environment variables,
let it can be override by gate_hook.
Change-Id: Ib33fffbaaea6abfbd9719e0712918d4ae65b3ee2
Partial-Bug: #1861469