Support canary releases

[BramKaashoek]

We have a set of sites (for EU, APAC, US, etc) configured in the same repository. We would like to be able to do canary releases.
Ideally we would roll out to 1 smaller region (APAC), perform checks (e2e tests, analytics based checks) and then roll out to the rest of the environments.

An initial idea would be to allow for the tagging of sites as canary, so that we can do
mach-composer apply --canary followed by CI/CD followed by mach-composer apply --non-canary. In an ideal world the canary release would get reverted on pipeline failure.