domains: Ability to customize the OU in which the machine account will be created

[dwlfrth]

We should add the ability to provide the OU in which the machine account will be created when joining the domain.

~$ net ads join --help
net ads join [options]
Valid options:
   createupn[=UPN]    Set the userPrincipalName attribute during the join.
                      The deault UPN is in the form host/netbiosname@REALM.
   createcomputer=OU  Precreate the computer account in a specific OU.
                      The OU string read from top to bottom without RDNs and delimited by a '/'.
                      E.g. "createcomputer=Computers/Servers/Unix"