admin: Fix default uri config option name s/admin_uri_default/uri_default

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>