nova-dhcpbridge's FLAGFILE environment variable is one of the few
remaining references we have to "flags".
Switching to use a new environment variable is easy, but we need
to take care that when you upgrade the code the compute and network
services might not be restarted immediately.
If this happens, the services may launch a new dnsmasq (so rootwrap
needs to continue to support FLAGFILE) or an existing dnsmasq might run
the lease script (so the script needs to support FLAGFILE).
Change-Id: Ieb55daa2507abb8327c7f5a2ef2f682740f31a5d
b33bcd58e5