Reserve 10 migrations for backports

Reserve 10 migrations so that we have the option of backporting fixes
that include a db migration to the stable/juno branch.  We did this
same thing to allow backports and stable/icehouse in
d65139e6f4885869f80ee510f95f733473c579b6.

The choice of 10 migrations is just a magic number.  We haven't used any
of the 10 reserved in previous stable branches, so 10 seems to be enough.

Implements blueprint backportable-db-migrations-kilo

Change-Id: I3e6d2579b22222e51134e517a97f9daa5855382c
Merge "Reserve 10 migrations for backports"