Add functions to modify LDAP attributes

[matthew-mcallister]

In particular, add a celery task.

At the moment, it fails its most useful function in that it won't work properly with the mail attribute which requires a privileged bind to read. This will probably have to be solved by scraping the output of ldapsearch.

It also may be confusing for create_ldap_entry_with_keytab to also modify LDAP entries now.

[matthew-mcallister]

Comments addressed.

[chriskuehl]

lgtm with a couple comments, no blockers

[matthew-mcallister]

Thanks for the extra comments. I took all the minor suggestions because I'm sure your python is more idiomatic than mine.