[Stable-Only] Enable build isolation for gnocchi

Disabling the build isolation breaks installing gnocchi, which
results in telemetry tests failing on stable branches as well
as the grenade test failing for ceilometer on master.

Change-Id: I00f504b80d087072a32e0e64e46b7dff4b2ff683
Signed-off-by: Jaromir Wysoglad <jwysogla@redhat.com>

inc/python

@@ -207,7 +207,11 @@ function pip_install {
     cmd_pip="$cmd_pip -c $REQUIREMENTS_DIR/upper-constraints.txt"
     # NOTE(stephenfin): stable-only change to disable build-isolation so that
     # we can pin setuptools on these old branches
-    cmd_pip="$cmd_pip --no-build-isolation"
+    # NOTE(jwysogla): Gnocchi doesn't install correctly without
+    # build-isolation, so disable it only if we aren't installing Gnocchi
+    if [[ ! "$@" =~ "gnocchi" ]]; then
+        cmd_pip="$cmd_pip --no-build-isolation"
+    fi
 
     $xtrace