Add base provides and conflicts columns to roles.

While we are at it, move all of the code that handled creating noderoles
from the Role model over to the NodeRole model as validation and
after_create callbacks on NodeRoles.
Add crowbar-docker-node and provisioner-docker-setup roles.

Currently, the roles don't actually do anything but exercise the
provides and conflicts stuff we just added.
Stop using the server flag
Update the docs
Finish zapping server flag
Merge pull request #99 from VictorLowther/role-provides-and-conflicts

+2 Add provides and conflicts stanzas to role definitions.