BUG-2385: introduce a shorter version of identifierForKey()

This takes advantage of the fact that the basePath is passed down to the
constructor -- so the subclasses can actually do a better job at
finiding the true base path.

Change-Id: I965e27ca6599bc606d8447acf7e6f2ddc1370ac4
Signed-off-by: Robert Varga <rovarga@cisco.com>