admin: Fix default uri config option name s/admin_uri_default/uri_default
    
    The original name 'admin_uri_default' was introduced to our code by commit
    dbecb87f. However, at that time we already had a separate config file for
    admin library but the commit mentioned above didn't properly adjust the
    config's option name. The result is that when we're loading the config, we
    check a non-existent config option (there's not much to do with the URIs
    anyway, since we only allow local connection). Additionally, virt-admin's man
    page documents, that the default URI can be altered by setting
    admin_uri_default option. So the fix proposed by this patch leaves the
    libvirt-admin.conf as is and adjusts the naming in the code as well as in the
    virt-admin's man page.
    
    Signed-off-by: Erik Skultety <eskultet@redhat.com>