Ignore "no such file" error when docker cannot find `resolv.conf`
Ignore "no such file" error when docker cannot find `resolv.conf`

`/etc/resolv.conf` is not an essential file in filesystem. (see
http://man7.org/linux/man-pages/man5/resolv.conf.5.html)

> If this file does not exist, only the name server on the local machine
> will be queried

It's baffling to users that containers can start with an empty
`resolv.conf` but cannot without this file.

This PR:
* ignore this error and use default servers for containers in `bridge`
  mode networking.
* create an empty resolv.conf in `/var/lib/docker/containers/<id>` in
  `host` mode networking.

Signed-off-by: Yuanhong Peng <605597f567e7188f3c817a57f8e09c6e857f9d0b@huawei.com>

medallia/libnetwork,mahak/libnetwork,vdemeester/libnetwork,abhi/libnetwork,fcrisciani/libnetwork,vdemeester/libnetwork,abhi/libnetwork,kolyshkin/libnetwork,docker/libnetwork,ityangchen/libnetwork,docker/libnetwork,medallia/libnetwork,mahak/libnetwork,ityangchen/libnetwork,kolyshkin/libnetwork,fcrisciani/libnetwork,mahak/libnetwork,medallia/libnetwork,vdemeester/libnetwork,docker/libnetwork,abhi/libnetwork,kolyshkin/libnetwork