TEST: Integration Test Setup for CERN CentOS 7

This adapts the cloud testing system and adds contextualisation scripts for CERN CentOS 7. Besides the actual contextualisation scripts the VM steering infrastructure can now deal with userdata in order to pre-configure the CentOS image (i.e. disable `requiretty` in `/etc/sudoers`). Furthermore the CentOS image does not provide a pre-installed `wget`, thus the platform-independent installation script is now able to fall-back to `curl`.

**Note:** Currently there is no AUFS enabled kernel for CentOS 7, hence we can only run the client tests.