Return team id when adding team

[jonathan-dorsey]

This pull request updates the Team code to return the computed id value for newly created teams. This is necessary in support of adding team resource functionality in the terraform provider. See grafana/terraform-provider-grafana#120

Also updated the test to use and validate the newly returned id value. While there, modified the Team code to use a value instead of a pointer. This is more consistent with some of the similar code in org, alert, user...

[jonathan-dorsey]

@tonglil @mlclmj - I believe this PR is ready for review. To retrieve and return the computed ID value, I followed the same pattern that was used for the organization to be consistent, but let me know if you would prefer something else. If we are worried about introducing a breaking change by adding the second return I could potentially create a separate method to add the team. This might be a better approach anyway, as it could accept a Team struct in the parameters and return a populated Team struct. Doing it that way would avoid changing the function signature any time some new field might be added to the Team in Grafana.