Add dhcp_build command

For OSU we'll need to put `dhcpd.conf` and all the `dhcpd.conf.*` files in whatever STAGE_DIR directory we set, then:

```
DHCP_TARGET_FILE = 'dhcpd.conf.data'
DHCP_CHECK_FILE = 'dhcpd.conf'
```

Some things still don't generate, and the names of the `avaya-hack` and `ip-phones-hack` classes in `dhcpd.conf.header` need to be changed to be compatible with Cyder's generated names (it appends the IP range to the vrf name).