Make dhcp-all-interfaces block all interfaces

Previously dhcp-all-interfaces was only blocking the first network
interface to come up. We add an instance stanza to make it run one
instance of generate-interfaces-file per interface. We then use flock to
serialize runs of generate-interfaces-file.

Fixes bug #1233577

Change-Id: Ib16bed6b37ce0789e315ef57e05ac561470a6f2a