Allow deployment of stable Conjur version

[john-odonnell]

Desired Outcome

Scripts for testing and examples should be able to run against the latest stable version of Conjur.
Scripts run against cyberark/conjur:edge for its advantages:

• bug discovery of unreleased changes in the Conjur server
• ensure that the OpenAPI description is kept in-phase with the server

Users that aren't actively developing the project should be able to run these tests/examples without
being made aware of possible bugs.

Implemented Changes

• bin/start_conjur can be configured in two ways:
  • --stable flag (prioritized)
  • global environment variable CONJUR_OPEN_SOURCE_IMAGE_TAG
• Scripts that call bin/start_conjur use either of the above methods
• util function ensure_conjur_up also ensures that a deployed version of Conjur is of the desired tag
• Examples always run against that latest stable version
• bin/test_integration and bin/test_api_contract scripts can be called with the --stable flag, but default to using Conjur's edge version
• CI pipelines run against cyberark/conjur:edge to maintain the advantages listed above.

Connected Issue/Story

Resolves #213

CyberArk internal issue link: N/A

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be
merged.

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: insert issue ID
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes