install-server: allow /sbin/ip for early_command

ip is installed under /sbin in at least trusty installers (and precise &
trusty systems). Leave /bin/ip as an alternative, just in case the older
releases need it.

This currently breaks trusty, as trusty installer has no ifconfig to
fall back to at all.

Change-Id: I0c4c9648802fe42cf44a4330cc9015b5d614b60c