Add code to do implicit binding of discovery and bootstrap roles.

At last, the node_role code is getting a bit of a workout.
Merge pull request #594 from VictorLowther/pull-req-master-f2dad2c18398d692a3bb26eb5af182c27059631e

Bind bootstrap and discovery roles to newly-discovered nodes by default. [1/1]
Make sure role_requires table is created before we start importing barclamps.
Update node_role and node models to make adding implicit barclamps work.
Make node_role.name vaguely useful for debugging.
Merge pull request #600 from VictorLowther/pull-req-master-6b623130e3c620b214baad504771b72ddf86cbb8

Fix up role_requires logic to make automatically adding implicit roles actually work. [1/1]
all BDD tests passing
merge w/ trunk, all tests passing