python: Use os.path.isdir(p) rather than p.isdir()

The latter is broken with Python 2.7 on Windows.  This changes the code to use os.path.isdir throughout.

Note that components/tools/OmeroPy/src/path.py uses os.path.isdir internally but I wasn't sure where this actually gets used, so users of this may have been converted to use os.path.isdir;  components/tools/OmeroPy/src/path.py looks like it was copying rather than calling, but I don't see any actual use of it. @cblackburn, is this correct?  What's the best thing to do here?  Do any of these changes break anything?

Testing: Should make bin/omero work with Python 2.7 / Ice 3.5 on Windows.  And there should be no regressions on any existing supported platform.