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
(cherry picked from commit 5b171d9800)

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

@@ -24,7 +24,10 @@ 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