Fix the validation of flavor_extraspecs v2 API

"create flavor_extraspecs" v2 API does not validate the data type
of a request body. If invalid parameter is passed, an internal error
happens. If many invalid requests come, a log file would be occupied
with traceback.

In addition, it does not validate the lengths of both key and value of
extra_specs. extra_specs are stored into the instance_type_extra_specs
table, and the key and value are defined as String(255).

This patch fixes the validation code from the viewpoint of data type
and key/value length.

Closes-Bug: #1264220

Change-Id: I195bd5d45a896e9b26dd81dab1e49c9f939b4805
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
Merge "Reserve 10 migrations for backports"
Merge "Fix the validation of flavor_extraspecs v2 API"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch