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/havana branch.  We did this
same thing to allow backports to stable/grizzly in
b52a2157bedba693c5da7dcb783b7a151769d6b1.

The choice of 10 migrations is just a magic number.  We haven't used any
of the 10 reserved for stable/grizzly backports, so 10 seems to be
enough.

Implements blueprint backportable-db-migrations-icehouse

Change-Id: I2c5f2e5a966cfffa4a3e54e17fde07a52295f658
Merge "Reserve 10 migrations for backports"