Investigate ways to make gateway parameters more dynamic

[pcuzner]

Currently gateway parameters are set in a conf file, and changing how the gateway behaves requires gateway restarts. To make things more complex in a production deployment the conf file is managed by cephadm, so any change needs to be done to a spec and a redeploy done to push the config to the gateway nodes, followed by a restart of each gateway.

Ceph moved away from rigid conf file a while ago, so this issue has been raised to examine whether it would be possible for the gateways to adopt a more dynamic approach to parameters.