Reserve 5 migrations for backports

Reserve 5 migrations incase the need arises to backport any
fixes that require a db migration in stable/juno.

We've never set this up in the past and we did run into a case
last cycle where we had to hack some things around to make it work
without the place holder.

Why 5?  Why not?  For as little as we touch the DB historically
this number should be more than sufficient.

Change-Id: I2830f6557c65c4c9b3056bb5ee72b17346e21823
Merge "Reserve 5 migrations for backports"