Fix stack inconsistency after app deletion

To expose IP addresses of Instance in MuranoPL we place new entry to
outputs section in Heat template that reference corresponding
OS::Nova::Server resource. During clean-up stage, if some application
was deleted from environment, we remove corresponding OS::Nova::Server
resource, but reference in outputs section stays.

Stack with reference to resource that is not present in template is
invalid - environment deployment fails.

This change add code that removes references to OS::Nova::Server when
corresponding Instance is deleted.

Attention: this change fix issue that can break murano-ci gate

Closes-bug: #1339630
Co-Authored-By: Stan Lagun <slagun@mirantis.com>
Change-Id: I74d32034969dd7f554d74fac87f407388e52dd7e
Merge "Fix stack inconsistency after app deletion"