Support control files

Allow templates to provide a control file (EG foo => foo.oac). The
control file must be a single YAML mapping. Directory control files are
not supported yet.

Provide a single valid key, allow_empty, by way of example. If
allow_empty is false, either the target file will not be created or if
there's already a file in place, it will be deleted.

Add requirement on 'six' library for Python 3.3 compatibility.

Advances blueprint oac-header
See spec I8fe7126d7261c1f106d7413bd31aa379e9ae5c3e

Change-Id: Ic6952688e4f8273885192bcd5eda8cb657f5a940
Merge "Support control files"