tempest: Conditionally populate http_image key.

The `[image]` section and `http_image` configuration key is
optional, only populate it if the caller has populated the
`TEST_SWIFT_IP` environment variable.

Signed-off-by: Frode Nordahl <fnordahl@ubuntu.com>

zaza/openstack/charm_tests/tempest/templates/tempest_v3.j2

@@ -55,7 +55,7 @@ api_v2 = false
 api_v3 = true
 {% endif %}
 
-{% if 'glance' in enabled_services %}
+{% if 'glance' in enabled_services and test_swift_ip %}
 [image]
 http_image = http://{{ test_swift_ip }}:80/swift/v1/images/cirros-0.3.4-x86_64-uec.tar.gz
 {% endif %}