ipatests: Add coverage for adding and removing sshpubkeys in ID overrides
Adds xmlrpc tests for:
- Adding a user ID override with sshpubkey
- Modifying a user ID override to contain sshpubkey
- Removing a sshpubkey value from a user ID override
https://fedorahosted.org/freeipa/ticket/4868
Reviewed-By: Martin Kosek <mkosek@redhat.com>