Make developers keypair configurable

Makes the developers keypair injected into new VMs configurable instead of hardcoded to 'vgl-developers'.

Requires the changes to portal-core from https://github.com/AuScope/portal-core/pull/12.

Merge this pull (noting changes to env.properties!) and update the dependency on portal-core to use the modified version of that from above pull request. Configure an existing keypair.

AC: Run a job and check that the new VM has been configured with the specified keypair.