handle older versions of ubuntu

In Precise and newer, the name has been changed to protect the innocent.
This evaluates the logic in the recipe instead of the attributes for no
other reason than it might be clearer to the reader.
update to older version handling

Moved conditional to attributes, converted some arrays of words to Ruby
Wordlists, and updated some spacing.
factor out the repository address to an attribute

In the instance where someone might want to work on an experiemental
branch of the wal-e codebase on their own fork, in conjunction with the
`revision` attribute, they can now choose where to source the code from.
add attribute to push more options to base backup

There are other commandline options for backup-push, such as
--cluster-read-rate-limit and --while-offline, et al.

Allowing an operator to specify any or all as a node attribute will
inject these into the cron job's command, in a forma like so:

    node.override[:wal_e][:base_backup][:options] =
    '--cluster-read-rate-limit 5000 --while-offline'

or any such string.

The inital value of `nil` will be represented by a blank, so the command
will have two spaces between `backup-push` and <data directory> if not
provided.
Merge pull request #13 from DataDog/miketheman/repo_source

factor out the repository address to an attribute
Merge pull request #14 from DataDog/miketheman/push_opts

add attribute to push more options to base backup
Merge pull request #12 from DataDog/miketheman/git_pkg

handle older versions of ubuntu