drivers/openstack: use more compact serialization for Keystone tokens

[majewsky]

Since the new format should have all redundancies eliminated from it, I figure that the roundtrip through gzip and base64 is not going to save much space (or could actually waste bytes if gzip cannot save more than the 33% that base64 adds). In any case, it's more computationally expensive, so I'm getting rid of it.

[SuperSandro2000]

We don't have benchmarks, do we?

[coveralls]

coverage: 80.221% (-0.04%) from 80.265%
when pulling 9e07c4e on keystone-compact-tokens
into de0ba0a on master.

[majewsky]

We don't have benchmarks, do we?

Not in the automated sense, no. Running a benchmark on this (in the https://pkg.go.dev/testing sense) would require instantiating the Keystone AuthPlugin, which would require having an OpenStack in scope for the tests. I will have a look at the CPU/RAM usage metrics after this is merged.