Add igwn-gwcelery metapackage

To support the low-latency group's event processor, we need to define an igwn-gwcelery metapackage that provides the system-level requirements for that workflow.

In private discussions with @leo-singer, those are described as

The OpenMP runtime
And also the OpenMPI runtime
(those are two different things)
I think that we use MPICH because it has the best cross-compatibility
And Redis
Specifically, redis-server and redis-cli