Make get_ips timeout poll configurable

This commit increases the default timeout used by lxc-start-ephemeral
from 5 to 10, and adds support for an LXC_IP_TIMEOUT override.

Patchset 2:
  - Previous patch used a command line arg.

Signed-off-by: John McFarlane <john@rockfloat.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>