Improve initial network settings to allow static IPs #18

This commit adds some small changes to the debian clean.sh script:

1. Remove the contents of /etc/resolv.conf. If DHCP is enabled, it would be set by DHCP, otherwise it should be set during provisioning. Having a wrong nameserver in /etc/resolv.conf causes SSH requests to the machine to hang until the lookup times out (which delays provisioning a lot)
2. Add the script /root/disable_dhcp_client.sh to the containers root. This script overwrites /etc/network/interfaces with  a version that does not touch eth0 and allows static configuration of eth0 via Vagrantfile.

An example how to set up static IP for debian is added to examples/Vagrantfile-debian-staticip