scylla_setup: use full path of ip command (additional fix)
scylla_setup: use full path of ip command (additional fix)

sbin directory might be not included in env PATH when command is executed from
remote, ip command can't be found.

Changes:
- `which ip` doesn't work with CentOS, use /sbin/ip

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests