Merge branch 'ansible_min_version' into 'master'

Update minimal ansible version to 2.6

See merge request openappstack/bootstrap!38

README.md

@@ -9,9 +9,9 @@ a Docker image) which contains utilities to manage your cluster.
 
 * You need to have ssh access (either as root, or as another user that can
   `sudo`) to the machines that will make up your cluster.
-* You need `ansible >= 2.5` installed on your workstation to run the bootstrap
+* You need `ansible >= 2.6` installed on your workstation to run the bootstrap
   scripts. Please install it using your system package manager.
-  In the case your system package manager doesn't provide `ansible >= 2.5` you
+  In the case your system package manager doesn't provide `ansible >= 2.6` you
   can install it via the python package manager like this:
 
     `pip3 install --user -r ansible/requirements.txt`

ansible/requirements.txt

-# ansible>=2.5 is needed for using the `helm` resource
-ansible>=2.5
+# ansible>=2.6 is needed for using the `k8s` resource
+ansible>=2.6

test/requirements.txt

-ansible>=2.5
+ansible>=2.6
 behave-webdriver>=0.2.2
 openshift>=0.8.6
 psutil>=5.5.0