Implement workaround for ansible unmount bug

It seems that unmount can falsely report success but removing it from
fstab seems to be a way to avoid that bug.

Example build failure caused by it:
http://logs.openstack.org/13/626613/1/check/tripleo-buildimage-overcloud-full-centos-7/e9d97b2/job-output.txt.gz

See https://github.com/ansible/ansible/issues/48313

Change-Id: I608ba6801f36f834d4e82c0438b41ea1e90620e8

roles/configure-swap/tasks/ephemeral.yaml

@@ -16,7 +16,11 @@
   become: yes
   mount:
     name: "{{ ephemeral_device }}"
-    state: unmounted
+    state: "{{ item }}"
+  with_items:
+    - unmounted
+    - absent
+  # ^ https://github.com/ansible/ansible/issues/48313
 
 - name: Get existing partitions
   become: yes