Override docker image version used by COA

[o-orand]

Is your feature request related to a problem? Please describe.
As paas-template user,
I want to be able to override docker image provided by COA,
In order to enforce a specific version to be more COA independant for these kind of update (e.g. terraform image)

Describe the solution you'd like
As it is not possible yet to use ((a-variable)) in task definition (see concourse/concourse#2182) without a credentials manager (credhub, vault), we need to look at other possibilities.
It is not very convenient to depend on a credentials manager to unit test concourse task, as fly execute doesn't support variable, we have to consider alternative, indeed. We may provide a default version in task definition, and override it when tasks are used in a pipeline using https://concourse-ci.org/task-step.html#task-step-image mechanism.

Additional context
Related to https://github.com/orange-cloudfoundry/paas-templates/issues/97