Allow model sharing across paas-templates

[o-orand]

Introduces a shared/models dir in paas-templates to allow sharing across root deployment.

Origin model could be described in deployment-dependencies.yml

---
deployment:
  bosh-ops:
    resources:
      template:
        base_path: ["shared/models/bosh"]
      secrets:
...

CC: @JCL38-ORANGE

[gberche-orange]

An alternative could be to split paas-template into distinct git repos, which can then included multiple times.

Ongoing discussions about leveraging native bosh runtime config as a way to share jobs across instance groups (altough runtime config expressivity remains limited)