join: Support site detection when --server is specified

join: Support site detection when --server is specified

When a new DC is joined to the domain, samba-tool would automatically
detect an appropriate site for the new DC. However, it only did this if
the --server option wasn't specified. The new DC's site got
automatically updated as part of the finddc() work, however, this step
gets skipped if we already know the server DC to join to.

In other words, if Default-First-Site-Name doesn't exist and you specify
--server in the join, then you have to also specify --site manually,
otherwise the command fails. This is precisely what's happening in the
join_ldapcmp.sh test, now that the backupfromdc testenv no longer has the
Default-First-Site-Name present.

This patch adds a new find_dc_site() function which uses the same
net.finddc() API (except based on the server-address rather than
domain-name). Assigning DEFAULTSITE has been moved so that it only
gets done if finddc() can't determine the site.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>