Update the canary rollouts to make the traffic increments a Substitution Variable

[dinagraves]

Proposal

Right now the canary.cloudbuild.yaml file says _TRAFFIC=$((${_TRAFFIC}+5)).

It's quite easy to go in and update this hardcoded value, but it would be better to have it be a substitution, so it can be different for both the staging and prod environments. One might want to do larger jumps in staging (even 100%) than one does in prod.

Should be something like _TRAFFIC=$((${_TRAFFIC}+${_TRAFFIC_INCREMENT})).

Should also set a default value for the increment by using the substitutions field like this.

Optional:

Update the pubsub_triggers.sh file to set different values for the substitution in the different environments at setup.