Implement juju deployment delete

[maxflintoff]

This implements the Delete action for juju_deployment.

Note This is known in Juju as destroy rather than delete so the client has a method Destroy that is called by the provider's delete command.

Tests are currently skipped pending read:

=== RUN   TestAcc_ResourceDeployment
    resource_deployment_test.go:20: Step 1/1 error: Error running post-apply refresh: exit status 1
        
        Error: not implemented
        
          with juju_deployment.this,
          on terraform_plugin_test.tf line 6, in resource "juju_deployment" "this":
           6: resource "juju_deployment" "this" {
        
--- FAIL: TestAcc_ResourceDeployment (4.01s)

[jon-ruckwood]

👍