Set backdoor port to a random port.

This commit enables the eventlet backdoor on a random port
for each service. For tempest coverage runs this allows the
coverage extension to connect to other services to enable
coverage reporting.

Change-Id: I1a2e9e9a8d14982276fd9683591563d4b5a65be8

devstack-vm-gate.sh

@@ -98,6 +98,12 @@ CINDER_SECURE_DELETE=False
 EOF
 fi
 
+if ["$DEVSTACK_GATE_TEMPEST_COVERAGE" -eq "1" ] ; then
+   cat <<\EOF >>localrc
+EXTRA_OPTS=(backdoor_port=0)
+EOF
+fi
+
 if [ "$DEVSTACK_GATE_POSTGRES" -eq "1" ]; then
         cat <<\EOF >>localrc
 use_database postgresql