This is basically revert of the [1] which was revert of the [2]
but now it should not break our CI jobs.
In the configure_for_func_testing script openvswitch is installed
from source. We need to set proper flag (Q_BUILD_OVS_FROM_GIT) which
is used in Devstack to tell Devstack to install it from source and
not from packages.
This patch also removes flag BUILD_OVS_FROM_SOURCE from the
configure_for_func_testing file as it was only used in that file
and was actually duplicating the Q_BUILD_OVS_FROM_GIT option used also
in Devstack.
[1] https://review.opendev.org/c/openstack/neutron/+/824750
[2] https://review.opendev.org/c/openstack/neutron/+/824750
Change-Id: I35715a047d23ed87312afd294cc898de7c164583