Mount additional Test Storage for Backend Tests

This extends the SLC6 backend test setup by creating two extra partitions on the remaining available virtual disk space. Simply extending the VM's boot partition (8GB on startup) is not sufficient, since its file system only provides roughly 500.000 inodes. This is far too less for our stress tests.

The new setup code grabs all the remaining space on `/dev/vda`, devides it into two equal halfs (with the current setup about 37GB in size) and creates two extra primary partitions. After that a reboot is performed (also because we exchange the kernel). The test script first formats the new partitions with ext4 allocating 10.000.000 inodes and mounts them on `/var/spool/cvmfs` respectively `/srv/cvmfs`.

Request for Comments! :-)