Use LXC v2.0.6 on CentOS

Both MariaDB and MemcacheD fail to start with
the newer versions available in the COPR, so
we're pinning to the older one to allow
development to continue until the solution is
identified.

Related-Bug: 1697531
Change-Id: If34240bf30f7f5bdc85d667d2ee79cca5b04dc45

releasenotes/notes/rhel-lxc-208-issue-3ea3ddf7c28c8e08.yaml

@@ -0,0 +1,6 @@
+---
+issues:
+  - |
+    Due to https://bugs.launchpad.net/openstack-ansible/+bug/1697531
+    the version of LXC used for CentOS/RHEL has been temporarily set
+    to v2.0.6.

vars/redhat-7.yml

@@ -27,7 +27,10 @@ lxc_hosts_distro_packages:
   - libseccomp
   - libselinux
   - libselinux-devel
-  - lxc
+  ## TODO(odyssey4me):
+  # Remove the version once the solution for the following bug is found.
+  # https://bugs.launchpad.net/openstack-ansible/+bug/1697531
+  - lxc-2.0.6
   - lxc-devel
   - lxc-libs
   - lxc-templates