Remove mysql version pinning to mysql-server-5.6

Ubuntu 18.04 LTS has 5.7 which is functionally the same, and removing the version pin should make it work in 99.9999% of installs...

roles/mysql/tasks/main.yml

@@ -2,7 +2,7 @@
 
 - name: mysql-server - installation
   apt:
-    pkg: mysql-server-5.6
+    pkg: mysql-server
     state: present
 
 - name: python-mysqldb - installation