Dev: Add a function not to grant a specific ticket at a specific site

This request is a proposal patch about exclusion of the ticket, adds a function which does not grant that ticket to the site specified by exclude_site in booth.conf.

**The example for which this function is needed.**
 * There are three sites. I want to operate two services (two tickets).
 * However, I want to operate only one service at one site.

**example of configuration**
```
site="192.168.203.101"
site="192.168.203.102"
site="192.168.203.103"

ticket="ticketA"
        exclude_site = 192.168.203.102
ticket="ticketB"
        exclude_site = 192.168.203.101
```
 * Then, I grant **ticketA** to **192.168.203.101** and **ticketB** to **192.168.203.102**.
 * By doing so, **only 192.168.203.103** becomes a **standby site** of ticketA and ticketB.