system: T5877: Shorten system domain-search config path

Shorten and simplify system domain-search config path from:

set system domain-search domain <domain1>

to:

set system domain-search <domain1>

This will shorten the path and also make consistent with domain-search
config in other places (like dhcp-server).

(cherry picked from commit f77bf573c608b6c09182e1bad4312c4dd1e5195e)