Add environment handling

This aims to seed/address #255 - rather than introducing a dedicated `Environment` parameter, I simply base the condition `IsLiveEnvironment` on the `CostCentre` being `logsearch-live` as recently decided - like so it addresses CI specifics within `composite-ci-r53.template`, but doesn't trickle down to the sub templates yet (can be extended once needed, but would require a more explicit `Environment` parameter or a more flexible definition of 'live' in case, e.g. via a map and/or list parameter).

@dpb587 - this addresses the one issue of separate stacks interfering with the live alarm by only using the `ClusterName` parameter for the `ClusterName` dimension value of the latter and `AWS::StackName` for all others - the composition obviously isn't completely transparent like so, maybe you have a better idea to refactor things?