Application spec should be verified before being deployed

[birgirst]

FIAAS supports multiple versions of application specs. When users upgrade their application specs from one version to the next it is easy to supply values that are not valid causing failures when deploying. By introducing a verification step in the form of linting of the application spec ahead of deploying it, it would be possible to raise issues more explicitly and give users feedback earlier in the process.

[oyvindio]

One way to verify/validate the application spec could be to add a complete openapi schema to the Application resource. Validation would be performed by the apiserver at resource creation time. This comment https://github.com/fiaas/fiaas-deploy-daemon/issues/38 has some details about what might be required to achieve this. I'm adding the link to it here so it is easier to find.