Support clouds.yaml service default microversions #3882

Summary

Gophercloud should preserve service default microversions from clouds.yaml.

Openstacksdk supports per-service settings like compute_default_microversion. We should support that pattern generally, not only for Compute.

Proposal

• preserve service default microversion settings from clouds.yaml
• start with compute_default_microversion as the first concrete field
• keep the API open to generalizing {service_type}_default_microversion

Examples

• Compute parser example: Support compute default microversion in clouds.yaml #3881
• Clientconfig example: Support compute default microversion in clouds.yaml utils#288