Travis: Add workaround for missing IPv6 support

Latest Travis CI image lacks IPv6 address on localhost. Add some
diagnostics and skip IPv6 tests in ipa-server-install when TRAVIS is
detected.

The hack will be removed as soon as it is no longer required to pass
automated testing.

https://pagure.io/freeipa/issue/7323

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Felipe Volpone <fbarreto@redhat.com>

.travis.yml

@@ -1,7 +1,13 @@
+# workaround for missing IPv6 address support
+# https://github.com/travis-ci/travis-ci/issues/8891
+sudo: required
+dist: trusty
+group: deprecated-2017Q4
+
 language: python
+
 services:
     - docker
-
 python:
     - "3.6"
 cache: pip
@@ -48,6 +54,13 @@ env:
                 test_xmlrpc/test_[l-z]*.py"
         - TASK_TO_RUN="tox"
           TEST_RUNNER_CONFIG=".test_runner_config.yaml"
+
+before_install:
+    - ip addr show
+    - ls /proc/net
+    - cat /proc/net/if_inet6
+#    - ip addr show dev lo | grep -q inet6 || (echo "No IPv6 address found"; exit 1)
+
 install:
     - pip3 install --upgrade pip
     - pip3 install pycodestyle