Add tftp_server pattern in ironic.conf

This is the fix for bug 1236623. Currently we used $my_ip as the
tftp_server, but we do need a dedicated config option especially
when the provisioning network and public network for nova are
different.

Change-Id: I4b619319cee55c974027a2c4667230a6a499628f
Partial-Bug: #1236623
Merge "Add tftp_server pattern in ironic.conf"