Add flexibility to find variables in multiple PP streams

[alistairsellar]

#109 assumes that all required variables are in the apm stream. When new ESMValTool recipes are added to CMEW, they may require variables from other streams. When a new recipe is added, we need to be able to:

• extend the list of streams for which request.json specifies time bounds
• specify different streams for individual variables

Questions:

• For the second point above, should we continue to specify each stream in variables.txt, or is there a better solution with CDDS (such as an alternative set of variable-stream mappings to CDDS's default)?