feat (proxy/admin): allow partition modifications

This patchset adds one new method to the partitions API
- PATCH /v1/partitions/{partition}

The partitions storage layer is also updated to support a simpler
update interface:
- update(self, name, **kwargs) [with kwargs in ('hosts', 'weight')]

The partitions transport layer is cleared of cruft and reuses JSON
handling utilities from the queues transport layer.

This patch implementation uses jsonschema to achieve self-documenting,
strict format validation of the incoming input.

The partition storage controllers are modified accordingly, as are the
tests. More tests are added for the partitions transport layer to
verify the correctness and to ease future refactorings.

Change-Id: Ifa92b1225421196f95131c2b74e3c07b07c4cfd4
Implements: blueprint placement-service
Closes-Bug: 1230841
Merge "feat (proxy/admin): allow partition modifications"