Allow dict's as schema definitions

All negative testing schema's will be moved to dict format. Therefore
the framework must support files and dict's. After all test are ported
to dict's file support will be deprecated.

Change-Id: Idb094f2817e7ed007a0ea47cf8f8602b5aeb6268
Partially-implements: bp api-schema-unification
Merge branch 'master' of https://github.com/openstack/tempest