add option to set management IP pattern
also add missing 'unconfigure system hostname'
for example pattern '!192.168.1.1' makes it possible that
WAN IP is selected instead of LAN IP
Signed-off-by: Daniel A. Maierhofer <git@damadmai.at>
[grammar and spelling fixes in commit message]
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>