Validate that python3 is ready for loguserdata

By default, ubuntu trusty images has python3 executable in
path, but we can't use that for executing loguserdata script,
because pkg_resources can't be imported. Now it's proposed to
try importing pkg_resources for validating readiness of
python3 for executing this script. If pkg_resources can't be
imported there is no other choice except using python2.

Change-Id: Icb4f58630016874eb40dd77590469fc5de6287e4
Closes-bug: 1617069

heat/cloudinit/loguserdata.py

@@ -12,7 +12,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 "true" '''\'
-if hash python3 2>/dev/null; then
+# NOTE(vgridnev): ubuntu trusty by default has python3,
+# but pkg_resources can't be imported.
+echo "import pkg_resources" | python3 2>/dev/null
+has_py3=$?
+if [ $has_py3 = 0 ]; then
     interpreter="python3"
 else
     interpreter="python"