Add Hiera override for distribution-specific data
Add Hiera override for distribution-specific data

Allow for Hiera data specific to an Ubuntu LTS release.

For example, to override Hiera values for the Precise LTS release, add
Hiera data to a `common.precise.yaml` file.

__NOTE__: If we use the values for Trusty as the class parameter
defaults, then override these in a `common.precise.yaml` file, we'll be
able to simply delete the `common.precise.yaml` file once we no longer
need to support it.

Use `common` as the prefix to indicate that this data will be overridden
by more specific Hiera data (e.g. class-specific configuration).

Use `::lsbdistcodename` as provided by Facter because we only ever use
LTS releases and each LTS release has a new `lsbdistcodename`.

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