Make developers keypair configurable

Developers keypair in CloudComputeService is configurable through the application context/environment. Falls back on old behaviour with hardcoded "vgl-developers" keypair name for backwards compatibility.

See the 'config-keypair' branch in my VHIRL-Portal repo (https://github.com/squireg/VHIRL-Portal.git) for example changes to config files and previously hardcoded references to 'vgl-developers'.

AC1: Running with this pull with an unchanged portal should behave as before (using 'vgl-developers').
AC2: Run this pull with my VHIRL-Portal modifications (or equivalent in another portal) and another existing keypair configured, and ensure job VMs are configured with the specified keypair.