Fix publishing_api logstream with ensure => present
Fix publishing_api logstream with ensure => present

This was added in d0cf038e08363d6a328c325e36d45f4ae8bb583a and has
been erroring in Preview on backends:

Error 400 on SERVER: validate_re(): true does not match ["^present$",
"^absent$"] at
/usr/share/puppet/production/current/modules/govuk/manifests/logstream.pp:35

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet