Add tags and import support to service offerings; fix zone_ids handling

[poddm]

Details

1. Add tags

• Added a Tags field to the common service offering model
• Added an optional tags attribute to the service offering schema
• Added an applyTags helper that sets tags on the create params, wired into
  Create for all three service offering resources

2. Fix zone_ids handling

• On read/update state (commonRead / commonUpdate), a returned zone id of
  "all" is no longer written into zone_ids; prior state is preserved so that
  a null or empty zone_ids (both meaning "all zones") doesn't produce a
  spurious diff
• commonUpdateParams now correctly serializes domain_ids and zone_ids by
  joining the set elements with commas instead of using Set.String(), and
  defaults zoneid to "all" when none are set
• commonCreateParams zone id handling cleaned up accordingly

3. Add import support

• Implemented ImportState (via ImportStatePassthroughID on id) for the
  fixed, constrained, and unconstrained service offering resources
• Added ImportState / ImportStateVerify steps to the acceptance tests for
  each resource

Files changed

• cloudstack/service_offering_constrained_resource.go
• cloudstack/service_offering_constrained_resource_test.go
• cloudstack/service_offering_fixed_resource.go
• cloudstack/service_offering_fixed_resource_test.go
• cloudstack/service_offering_models.go
• cloudstack/service_offering_schema.go
• cloudstack/service_offering_unconstrained_resource.go
• cloudstack/service_offering_unconstrained_resource_test.go
• cloudstack/service_offering_util.go