Order deploy tasks by host groups

Riff-Raff needs to be more intelligent about the order it processes hosts.  Historically, it has used the
ordering in the deployinfo file.  Now we finally make use of the 'group' tag on each host.  This tag
indicates a logical cluster of nodes of which only one should be taken out of service at any one
point in time.

The tasks are now reordered to rotate through each of the groups so we alternate between data centers, regions
or availability zones as appropriate.

The hosts in the deployment info tab are shown in their expected execution order.
Merge pull request #101 from guardian/sh-order-deploy-tasks-by-group

Order deploy tasks by host groups