do drake tags properly

[potash]

Currently drake tag names (which come from either step class names or step names) get included on the line of each step. For example if there are two A steps their drake lines look like:

%A, A/1/target <- A/1/step.yaml, ...
%A, A/2/target <- A/2/step.yaml, ...

For rebuilding all A steps then it makes more sense to write a drakefile like:

A/1/target <- A/1/step.yaml, ...
A/2/target <- A/2/step.yaml, ...
%A <- A/1/target, A/2/target