Creates and attaches second HDD

This commit creates and attaches a second disc drive to machines created using
Vagrant within this repository. This is done to ensure that the second drives
are present as Puppet will try to mount it as an LVM/ext4mount drive on
start-up.
Adds *.vdi to .gitignore

Ensures that we don't accidentally commit VDI disk to repo.
Add mounts manifest

This commit adds a mounts.pp manifest, which configures and mounts LVM disks.
It always checks for the existence of a `backup-data` dir at /dev/sdb, which it creates if required.
Removes extra nodes from Vagrantfile

Since we are unlikely to need to test a multi-node environment with off-site
backups, the two additional nodes have been removed from the Vagrantfile.
Adds alphagov/{lvm,ext4mount} module to Puppetfile

This commit adds the 'alphagov/puppetlabs-lvm' and 'alphagov/puppet-ext4mount'
modules to the Puppetfile, and then creates a Puppetfile.lock file using
librarian-puppet. Both of these modules are used within this repo.
Adds class to Hiera

This commit adds the base::mounts class (specified in mounts.pp) to Hiera to
allow for autoloading of this module.