r/wal-g-pg: Deploy WAL-G for PostgreSQL

This role installs `wal-g` from the DCH Yum repository, and creates a
configuration file for it in `/etc/postgresql`.  Additionally, it
installs a custom SELinux policy module that allows `wal-g` to run in
the `postgresql_t` domain (i.e. when spawned by the PostgreSQL server).

roles/wal-g-pg/handlers/main.yml

@@ -0,0 +1,3 @@
+- name: load wal-g-postgresql selinux module
+  command:
+    semodule -i /usr/local/share/selinux/wal-g-postgresql.cil