Honor default home directory and login shell in user_add
The homedirectory argument had a default_from '/home/<name>', ignoring
the ipahomesrootdir config setting. This patch removes that default,
and adds a test case for ipahomesrootdir.
https://fedorahosted.org/freeipa/ticket/2332
The login shell had the same problem. Again this patch removes the
client-side default and adds a test.
Building the home directory from the default is changed to use
posixpath.join instead of string formatting and ad-hoc cleanup,
and to use '/home' instead of failing when the ipahomesrootdir
setting is not present for some reason.