Should be `self.create_tag`. `bin/omero tag create` currently breaks with

``
Traceback (most recent call last):
  File "dist/bin/omero", line 125, in <module>
    rv = omero.cli.argv()
  File "/Users/sebastien/code/ome/openmicroscopy/dist/lib/python/omero/cli.py", line 1195, in argv
    cli.invoke(args[1:])
  File "/Users/sebastien/code/ome/openmicroscopy/dist/lib/python/omero/cli.py", line 745, in invoke
    stop = self.onecmd(line, previous_args)
  File "/Users/sebastien/code/ome/openmicroscopy/dist/lib/python/omero/cli.py", line 814, in onecmd
    self.execute(line, previous_args)
  File "/Users/sebastien/code/ome/openmicroscopy/dist/lib/python/omero/cli.py", line 894, in execute
    args.func(args)
  File "/Users/sebastien/code/ome/openmicroscopy/dist/lib/python/omero/plugins/tag.py", line 401, in create
    tag = create_tag(args.name, args.desc)
NameError: global name 'create_tag' is not defined
``