baremetal: Allow BM_SECOND_MAC to be unset

Currently DevStack registers the second nic of the baremetal
node even if BM_SECOND_MAC is not set or empty. However
an interface with an empty mac address causes dhcp to fail
(bug 1238595). And such operation will get to return a error
after the bug is resolved. So we should not register the
second nic if BM_SECOND_MAC is not set.

Related-Bug: #1238595
Change-Id: Ib3cc77686b72311403ccacbd70ae9cf43e6eb4c9
Merge "baremetal: Allow BM_SECOND_MAC to be unset"