Add on/off switch for govuk_env_sync
Add on/off switch for govuk_env_sync

- So far our hiera config only allowed to add things, rather than to
  remove them

- We want to be able to turn off data sync to preserve the state
  of integration and/or in some cases, e.g. autumn budget prep

solo: da39a3ee5e6b4b0d3255bfef95601890afd80709@schmie

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