Allow scripts to deploy Conjur either as latest or edge versions

[john-odonnell]

Is your feature request related to a problem? Please describe.

Currently, docker-compose.yml hardcodes the Conjur image as cyberark/conjur:edge.
This has advantages, because it leads to bug discovery (cyberark/conjur#2438) in unreleased content,
and keeps the OpenAPI description in-phase with the server itself - however, external users should be
able to use the tooling around the OpenAPI description against the latest stable version, to avoid errors
in unreleased content that they don't need to be aware of.

Describe the solution you would like

Scripts should be able to choose between running against Conjur's edge or latest images.
CI would still test against edge - this would maintain the above stated value.

It might be a good idea for the scripts' to default to testing against conjur:latest, so users
running the scripts aren't experiencing unreleased content unless intentionally triggered.

Changes should be made to bin/start_conjur to allow version selection. This could be:

• a flag, such as --stable or --edge
• a global environment variable, such as CONJUR_VERSION or CONJUR_IMAGE_TAG

These changes have to propagate upwards to at least the following:

• bin/util function ensure_conjur_up, which would in turn propagate to
  • examples/python/start
  • examples/ruby/start
  • examples/kong/start
• bin/dev
• bin/test_integration

Describe alternatives you have considered

A clear and concise description of any alternative solutions or features that may be related to this that
you have considered.

Additional context

Add any other context information about the feature request here.