Allow multiple nodes in a rack

Use a FormsetDataTable in Rack's create and edit workflows to allow
creating or updating multiple nodes at once. This is a stopgap solution
needed before we completely rework this workflow according to the new
wireframes. There is a problem left:

* There is currently no way to update the existing nodes in nova
  baremetal, so editing the fields of existing nodes has no effect.

Closes-Bug: #1234245
Implements: blueprint rack-nodes-1-to-many
Change-Id: I27f5e9df97af6ba49acf9702fac17a0a883c6930
Merge "Allow multiple nodes in a rack"