Validate resource attributes for preallocated hosts before executing leases

Summary: This prevents issues when the user hasn't provided the appropriate attributes for a preallocated host.

Test Plan: Attempted to lease against a resource with omitted attributes, got an exception thrown before any SSH commands occurred.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7695