Select Microcks instances from Backstage Location entity list

[davgordo]

The API Specification golden path requires a parameter value representing the Microcks service URL to which a new mock API instance should be applied. The value is a free-form string, but would be better as a drop-down list which is dynamically populated with valid URLs. Limiting the possible user-provided values to only valid service URLs will make the template more secure and less error-prone.

Implementation notes:

• Current state, the URL value is pre-populated, but this requires updating this repository every time there is a new cluster wildcard domain, it also means this repository only integrates with Microcks on one cluster at a time