Pass context.Context to the store implementations

Pass context.Context to the store implementations

- this service defines a Store interface with implementations
  for mongo and consul
- enhance the methods of this interface to require
  a context.Context parameter
- this is the first step in being able to pass the (request) context
  from the handler to the mongo driver
- the context propagation is needed for the tracing instrumenations
  to pick up the overall trace context
- bump snapshot version to 0.12.0-SNAPSHOT

Issue-ID: MULTICLOUD-1537
Change-Id: Ibfcdbab6b46bc18a425a8abb3835c35371044bbc
Signed-off-by: Fiete Ostkamp <fiete.ostkamp@telekom.de>