tests: bust md5 of object not footers
tests: bust md5 of object not footers

The test test_PUT_ec_fragment_quorum_archive_etag_mismatch
busts the md5 in server.py, so it ends damaging the md5 of
footers instead of the fragment archive. It appears that the
intention of the test was to check the integrity verification
for fragment archive, so change the test to bust diskfile.py
instead.

Change-Id: I54a203bb637d5f5814e8df2b4297758b0b72adac

openstack/swift,tipabu/swift,swiftstack/swift,openstack/swift,swiftstack/swift,tipabu/swift,openstack/swift,openstack/swift,tipabu/swift,tipabu/swift,swiftstack/swift