[ctdb] use lowercase plugin class name

sos-2.2 does not allow capitalised plugin class names:

sosreport (version 2.2)

plugin ctdb.py does not install, skipping
Traceback (most recent call last):
  File "/usr/sbin/sosreport", line 23, in <module>
    sosreport(sys.argv[1:])
  File "/usr/lib/python2.6/site-packages/sos/sosreport.py", line 453, in sosreport
    pluginClass = importPlugin("sos.plugins." + plugbase, plugbase)
AttributeError: 'module' object has no attribute 'ctdb'

> /usr/lib/python2.6/site-packages/sos/helpers.py(39)importPlugin()
-> return getattr(plugin, name)
(Pdb)

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>