Add support for host constraints

[dgrnbrg]

This should be for things like "only on hosts w/ a specific attribute". This will enable things like GPU or machine class aware scheduling.

This will need to be added to the client-facing API, as well as to the scheduler & db.

[dgrnbrg]

@shamsmahmood Can you comment on what you did at the Mesoscon Europe hackathon on this?

[shamsimam]

I added support in the Cook json api to accept host constraints as key value pairs. The database schema has also been updated so that these constraints are stored in the database. The unit tests in api.clj should also help explain what has been done (https://github.com/shamsmahmood/Cook).

[dgrnbrg]

#69 has the API side of this implemented. Once #84 is merged, it should be easy to complete the integration, as #84 includes an API for adding constraints.

[DaoWen]

@shamsimam - I think this is done now, right? Please close this if you agree.

[shamsimam]

Was implemented in #284