Jenkins authenticates with session token (authn-k8s support)

[AndrewCopeland]

Currently Jenkins supports authentication via the conjur api key, however if jenkins is being run in k8s with the conjur side car container a session token is provided to the jenkins container.

We need the ability to authenticate using this session token rather than an api key.

The conjur configuration should contain a field for 'Session Token File' and if this field is populated then the jenkins plugin should use the session token to retrieve the credential.

This also means that the conjur-credential-plugin does not need to authenticate since it is already authenticated.

[guygiat]

In addition we will need to think how can we keep the segregation of duties that we have now using folders in Jenkins with different API keys.
Using the authn-k8s/azure/iam we have support for only one host at the moment.