Use simplified relationships for all services & vendors

[chadwcarlson]

Closes #222

Updating Upsun and Platform.sh was a bit complicated for the tests and expected types.

Currently:

• Upsun uses explicit relationships, but of incorrect format. (see issue linked above for details)
• Platform.sh uses legacy relationships

This PR (for now) updates both Platform.sh and Upsun platformify commands (when a service is chosen) to use the simplified relationships format.

[chadwcarlson]

Stuck modifying tests for empty string and expected type in answer_test.go

=== RUN   TestPlatformifier_Platformify/Django
    platformifier_test.go:509: Platformifier.Platformify() validation error = validation failed for .platform.app.yaml: relationships.db: Invalid type. Expected: string, given: null
        relationships.mysql: Invalid type. Expected: string, given: null, wantErr false
=== RUN   TestPlatformifier_Platformify/Generic
    platformifier_test.go:509: Platformifier.Platformify() validation error = validation failed for .platform.app.yaml: relationships.mysql: Invalid type. Expected: string, given: null, wantErr false
=== RUN   TestPlatformifier_Platformify/Laravel
=== RUN   TestPlatformifier_Platformify/Next.js
=== RUN   TestPlatformifier_Platformify/Strapi
=== RUN   TestPlatformifier_Platformify/Flask
=== RUN   TestPlatformifier_Platformify/Express
--- FAIL: TestPlatformifier_Platformify (0.05s)
    --- FAIL: TestPlatformifier_Platformify/Django (0.01s)
    --- FAIL: TestPlatformifier_Platformify/Generic (0.01s)
    --- PASS: TestPlatformifier_Platformify/Laravel (0.01s)
    --- PASS: TestPlatformifier_Platformify/Next.js (0.01s)
    --- PASS: TestPlatformifier_Platformify/Strapi (0.01s)
    --- PASS: TestPlatformifier_Platformify/Flask (0.01s)
    --- PASS: TestPlatformifier_Platformify/Express (0.01s)

[akalipetis]

LGTM!

@chadwcarlson I fixed the failing tests and restructured the code a bit, to handle the new logic.

Let me know if you're OK with the updates and I'll be happy to merge 🎉