Add a nova keypair constraint

The patch adds a new custom constraint validating nova keypairs, and
uses it in the Instance and Server resources.

blueprint param-constraints
Co-Authored-By: cedric.soulas@cloudwatt.com
Change-Id: I98ef4dd94a165da38e188d4a5d7c0b5d8ff76fef
Merge "Add a nova keypair constraint"