Add support for descriptor includes

[wtschreiter]

TL;DR

Add support to reuse descriptors by including them into other descriptors.

UseCase

We do have scenarios there we to deploy the same application stack on different rancher managed clusters.
Depending on the abilities of the cluster we have to handle stuff as persistent volumes and storage classes very different. But the set of applications is at the same time very similar.
Currently we have the choice of duplicating the full descriptor or implementing some template logic for the different clusters. Both not very maintainable.

Solution Proposal

• Have includes declared in the metadata section of the descriptor.
• Have the resulting descriptor object constructed from all includes and the including descriptor
  • The later the include the higher the precedence for duplicate names.
  • The including descriptor has the highest precedence.
• Create a error response on a detected include cycle.

api_version: v1.0
kind: Project
metadata:
  name: sample-project
  includes:
    - relative-include.yaml
    - /tmp/ablsolute-include.yaml