pmdaprometheus: sort os.walk() URLs for consistent indom cluster numbers
pmdaprometheus: sort os.walk() URLs for consistent indom cluster numbers

XFS and EXT4 may return a different file order from os.walk() when
traversing the urls.d directory, so we need to sort the file list in
order to get consistent indom cluster numbers for the metric sources.
Mostly affects QA on different systems with different filesystems.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp