Add 'fixpyramids' command to admin.py

'bin/omero admin fixpyramids' will now remove any 0-length pyramid files
that do not have a corresponding lock file or temp file.  This allows
administrators to force regeneration of pyramid files that could not be
created before (e.g. due to OOMs as a result of the server
configuration).

See ticket #11395.
Refactor fixpyramids to act more like cleanse
Refactor to use os.walk
Merge pull request #1608 from sbesson/rebased/develop/11395-fixpyramids

11395 fixpyramids (rebased onto develop)