cli, api: usability improvements

[epuertat]

Few suggestions on usability:

• API (gRPC proto) should improve internal consistency:
  • Arguments, field names:
  • Subsystem NQN is named in 3 different was on different methods: subsystem_nqn, subsystem and nqn.
  • We have anagrpid, load_balancing_group (are they the same thing?).
  • Methods should follow a consistent naming:
  • currently we have namespace_add (which is consistent with CLI), but list_namespaces. All methods should adopt the same convention, preferably <object>_<verb>.
  • Remove unneeded params:
  • Is namespace uuid really required? Compared to having nsid (indexes, like 1, 2, 3...), it looks like a unfriendly param. That's ok for metadata, but most namespace calls accept optionally uuid and nsid.

### Tasks
- [ ] Arguments/fields should improve internal consistency
- [ ] Methods should follow a consistent naming
- [ ] Remove/param clean-up

[gbregman]

@epuertat uuid is not a required parameter in any namespace command. If the user doesn't supply it one will be generated for him. We still allow to use in namespace commands as the namespace identifier. If the user find it unfriendly he can totally ignore it. There is no need to remove it from gRPC. People using API and not the CLI might find it usefull