Upgrade migrate_ldap process.

We now support importing data from any LDAP server to another LDAP
server without making changes to source LDAP server.

Note we don't delete the source unless conflict. This might cause
problems in certain situations that I can't think of right now. Will
address this if/when it becomes an issue.

Change-Id: Ie9b5a8f5f316412f94a2d91b5b8ef34c097b82c2