Rework user_formulas to support a map of {formula_name}: {formula_source}

[lorengordon]

Right now, effectively, we use formula_termination_strings to determine the formula name (and thus the name of the formula directory in the <salt_srv> root). That gets to be rather hacky. Here's the current config definition:

      formula_termination_strings:
        - -master
        - -latest
      user_formulas:
        #To add other formulas, make sure it is a url to a zipped file as follows:
        #- https://s3.amazonaws.com/salt-formulas/systemprep-formula-master.zip
        #- https://s3.amazonaws.com/salt-formulas/ash-linux-formula-master.zip
        #To "overwrite" submodule formulas, make sure name matches submodule names.

Instead, change the spec for user_formulas to be a map of the formula name and the formula source:

      user_formulas:
        #systemprep-formula: https://s3.amazonaws.com/salt-formulas/systemprep-formula-master.zip
        #ash-linux-formula: https://s3.amazonaws.com/salt-formulas/ash-linux-formula-master.zip

This would deprecate formula_termination_strings.