Dynafed-storagestats

REMOVED: wglc-min-schema sample file as it was not enough information.
ADDED: wlcg-schema sample file according to WCLG SRR.
ADDED: Dynafed version and update time for the WLCG space-usage.json file.
ADDED: helper get_dynafed_version() function to query rpm from dynafed version.

CHANGED: Used flake8 to format the code according to convention.
FIXED: wlcg example and output where the top storage service was an array
and should not.

CHANGED: Keys from the received 'Metadata' dictionary from an object are
converted to lowercase to fix discrepancy in capitalization from
different S3 implementations.
FIXED: Exception name from AssertError to AssertionError.

ADDED: type=str.lower to make sure checksums and checksums types are always
lowercase.

ADDED: Requirement of PyYAML.

ADDED: '--rucio' flag to 'reports filelist' that sets the prefix='rucio' and
delta='1' used for rucio consistency check filedumps.
ADDED: '--wlcg' flag to 'reports storage' that generates a json file according
to WLCG's storage accounting format from specified site schema file.
CHANGED: The reports file list now removes the chosen prefix from the paths.