[client] Disable "add/update" without selection monitoring server type.

We should disable "add" or "update" button without selecting monitoring
server type.

Signed-off-by: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Merge pull request #752 from project-hatohol/disable-add-button-when-opening-dialog-20141110

[client] Disable "add/update" without selection monitoring server type.