oo-accept-node: Handle non-existence of /sbin/ip

Modify check_node_public_resolution to rescue from Errno::ENOENT when
running /sbin/ip in case the command does not exist.  This change makes
check_node_public_resolution consistent with the find_ext_net_dev test.

This commit fixes bug 1121224.
Merge pull request #5624 from Miciah/bug-1121224-oo-accept-node-handle-non-existence-of-slash-sbin-slash-ip

Merged by openshift-bot