Add playbook for ssh banner warning users of SVN migration

Added a playbook that configures a ssh banner for the 'svn' account,
letting users know about the upcoming migration of the SVN repository
and the changes they need to do. This is added to both 'code' and
'leptone', so that after the migration to leptone, people who are unable
to login (eg. because they still didn't put keys in gitlab) will also get
the message.

Ref: T12973