Operator-triggered model update of one or more dedicated instances

[gberche-orange]

• As a service operator
• In order to
  • ensure security patching of vulnerable database deployments
  • rationalize versions among service instance fleet (e.g. to reduce support costs)
• I need to be able to trigger an upgrade of one or more dedicated service instances, i.e.
  • service branch is rebased against current reference branch
  • service instance symlinks are regenerated and include newly available operators in model

This is a distinct issue requested by @poblin-orange w.r.t to the more generic #40

Implementation steps following #343

• Rebase the service instance branch against the reference
• Override all COAB generated files (i.e. symlinks to model files) including coab-vars.yml file (introduced in Service plan and params support #29) containing
  • desired state
  • previous state (obtained from loading in memory content of coab-vars.yml on disk)
• git push