The default MariaDB apt repo was changed from using HTTPS to HTTP in
change I67a9fb48ee08866bd59d70730e62b7da28f81125.
Add an upgrade playbook between M->N to ensure that existing repos are
removed to avoid a 'Duplicate sources.list entry' error when the new
default is put in place.
Change-Id: If9693d61904cce9c57fd7f5fad68b57f2159955d
7a70d25a88