Updated openstack/openstack
Project: openstack/nova  7b170eeaef05db4da5bbddc7bb48ebd744cdb75f
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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/icehouse branch.  We did this
same thing to allow backports to stable/grizzly in
b52a2157bedba693c5da7dcb783b7a151769d6b1 and stable/havana in
ab2c467da951071a8aac4eb6ca032371c69053ab.

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-juno

Change-Id: Ifdeba76809a705413bacee571d6d0d43ce1f062c