Drydock blueprint for preallocated remote hosts

Summary:
This adds a Drydock blueprint for preallocated, remote hosts. This will be used by the Harbormaster interface to allow users to specify remote hosts that builds can be run on.

This adds a canAllocateResource method to Drydock blueprints; it is used to detect whether a blueprint can allocate a resource for the given type and attributes.

Test Plan:
Ran:

bin/drydock lease --type host --attributes remote=true,preallocated=true,host=192.168.56.101,port=22,user=james,keyfile=,path=C:\\Build\\,platform=windows

and saw the "C:\Build\<id>" folder appear on the remote Windows machine. Viewed the lease and resource in Drydock as well.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, jamesr

Maniphest Tasks: T4111

Differential Revision: https://secure.phabricator.com/D7593