Enable `service_instance_sharing` flag
Enable `service_instance_sharing` flag

This flag enables some of our tenants to speed up the creation of PR
test instances to aid in their CI/CD workflow.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf