Slave info: expose the buildslave info dictionary as API

We added an 'info' dictionary to the database for buildslaves. So far, we only had 4 items in it (admin, host, access uri, version). This patch makes the dictionary available for users to use.

* Buildslave.updateSlaveInfo and . getSlaveInfo for the API to update it from custom steps
* support for Interpolate("%(slave:XXXX)s")
* SetSlaveInfo buildstep to correspond with SetProperties
* unit tests for Buildslave Status; yes it's going away in nine, but may as well have tests for master. I was changing some things there, so figured I should TDD it first.