Enabling Service Instance Sharing

[thehandsomezebra]

Is your feature request related to a problem? Please describe.
CloudFoundry allows for services to be shared across orgs and spaces -- however this requires that the service broker is explicitly enabling service instance sharing by setting a flag shareable: true. At this time, the AWS ServiceBroker does not pass this variable.

Describe the solution you'd like
I will make a PR for this change. It will be backwards compatible since it is technically without the value, it is being defaulted to false.

Describe alternatives you've considered
Using a fork with this change allows the service broker to pass a template with shareable: true very well.

Additional context
Please see https://docs.cloudfoundry.org/services/enable-sharing.html for information