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.
Fixes syntax

Adds...
- soft-tab spacing
- documentation
- a colon to separate array of title names from params
- an ensure, to ensure packages are installed
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.