Environment variable improvements
This is a small bugfix release taking care of some issues that came up after the release of 0.52.0.0.
- support multiple environment variables properly
- don't store the IECORE_LOG_LEVEL in the user-facing environment variables. Instead GafferDeadline will default to IECORE_LOG_LEVEL=INFO when submitting unless a different IECORE_LOG_LEVEL is set by the user.