Adding an option "global-parameters" for dhcpv6-server
For ipv4 it looks like
```
vyos@r-roll01# set service dhcp-server global-parameters
Possible completions:
<text> Additional global parameters for DHCP server. You must
use the syntax of dhcpd.conf in this text-field. Using this
without proper knowledge may result in a crashed DHCP server.
Check system log to look for errors.
```