make lxc-net hermetic w.r.t. existing dnsmasq config

make lxc-net hermetic w.r.t. existing dnsmasq config

lxc's dnsmasq will try to read system's dnsmasq if `--conf-file` is not
specified. This is likely not desirable, as lxc's dnsmasq should be
self-contained.

On my system the conflicting options are `--bind-interfaces` and
`--bind-dynamic`, since the same host is doing other DNS-y things
unrelated to lxc.

This is an incompatible change, since lxc's dnsmasq will stop honoring
system's `/etc/dnsmasq.conf`, and some systems may be relying on it.
Given that, I believe it should not depend on it by default, since
dnsmasq is lxc's implementation detail. However, if the user desires,
the old behavior could be brought back by setting
`LXC_DHCP_CONFILE=/etc/dnsmasq.conf` in `/etc/default/lxc-net`.

Signed-off-by: Motiejus Jakštys <motiejus@jakstys.lt>