new constraints middleware

Adding a constraints middleware that allows different constraints for a
Swift-on-File storage policy.

Currently, Swift allows only one set of object naming and metadata rules
per cluster. As new storage policies are implemented, it is possible that
different storage back-ends have different set of rules on how objects should
be created for that specific storage system. Swift-on-File has different rules
as objects name are mapped to a POSIX filesystem directory structure.
For example: no names with double slashes or ending with slashes are allowed.

At first a solution was proposed to include a generic patch in the upstream
swift code, but after discussing with the Swift community it became clear that
it would be better to have a global set of constraints that covers the whole
cluster and if a specific storage policy has specific constraints than it
should have its own middleware.
Link to patch for reference: https://review.openstack.org/#/c/113325/

Change-Id: I323ead5d98bf5c087930ccf446d3e8d83075e584
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Merge "new constraints middleware"