Inhibit docker restarting when no configuration change

I have changed to inhibit docker restarting when no configuration change to make docker containers alive as many as possible. This change is related to #85, but this can only solve a part of the issue.

I have tested with a [Vagrantfile](https://gist.github.com/otahi/25021510386a12e765bd). The test result is OK(I avoided CentOS 6.5 because docker does not work with newest package).
```
[otahi@otahiair vagrant-proxyconf-test]$ for vm in centos7 ubuntu14.04 boot2docker ; do echo -n $vm, ; vagrant ssh $vm -c 'sudo
 cat /proc/`pgrep docker|head -n1`/environ | grep -c http_proxy' 2&> /dev/null ; done
centos7,1
ubuntu14.04,1
boot2docker,1
[otahi@otahiair vagrant-proxyconf-test]$
```