The pre test hook for gate tests has a faulty test
for whether the PYTHON3_VERSION variable is set and
as a result puts a blank value for this variable in
local.conf when it is not set instead of leaving it
out of local.conf altogether. This causes jobs
running with USE_PYTHON3=True and no explict version
setting to actually run manila under python 2.7 because
that is what devstack itself defaults for pip installs
if the version is not set (or auto-detected in devstack
when local.conf does not contain the PYTHON3_VERSION
variable at all).
So correct the condition in our pre-test hook so that
it only inserts a PYTHON3_VERSION in local.conf if
that version has been supplied.