Minor refactor of storage policy helper functions

Pulled from a larger pending review where it became clear that
we didn't need to keep adding get_data_dir(index) type routines
so this patch makes a generic function in the policy module that
constructs strings given a base and a policy index then others
can use it directly or as a partial (as is done with get_data_dir()
for example)

Change-Id: Ia3bed58f4b8d52f44b8ab3e8fad8014231c19c50
Implements: blueprint storage-policies
Merge "Minor refactor of storage policy helper functions" into feature/ec