Clarify and fix static leases and autohosts with disabled DHCP servers

[ainar-g]

1. Configure and enable the DHCP server.

2. Add a valid static lease, for example myhost.

3. Make a DNS query for myhost.lan and get a successful response.

   So far so good.

4. Disable the DHCP server without restarting AGH.

5. Make a DNS query for myhost.lan and get a successful response.

   I'm not sure that this is intended.

6. Restart AGH.

7. Make a DNS query for myhost.lan and get an NXDOMAIN.

   I think this is intended, but this is inconsistent with the result from the request after disabling the DHCP server and before restarting AGH.

@ameshkov, I think that we should respond with NXDOMAIN on step 5 here. What do you think?

[ameshkov]

Probably you're right, we should respond with NXDOMAIN