Optimize imports of users and groups

[obscurerichard]

When I asked @rcritten for permission to publish this derived work, he gave some excellent advice on optimizing the imports:

@rcritten:
As an aside, adding group members one at a time may bring with it a big
performance penalty if you have a group with a lot of members.

@obscurerichard:
Thanks! I had it the other way (group members comma separated) initially but I got
an error code from the "ipa group-member-add" when I first tried it that way so I
fell back to this simpler but slower method. I may take a 2nd pass at doing it that
way to see if it worked anyway.

@rcritten:
You should be able to pass them in with multiple --users options (or the
API equivalent).

The batch command is another alternative. You can do the single-user
adds as one batch command and execute them all at once without all the
round-trips. I've tended to batch up the commands in groups of 50 or 100.

It looks something like this:

batch = []
args = [groupname]
kw = {'users': user}
batch.append({'method': 'group_add_member', 'params': [args, kw]})

When you've collected all the users you want, execute the command with

api.Command.batch(batch)