This change splits out discovery fields from JoinConfiguration by performing
the following changes:

- Introduce a BootstrapTokenDiscovery structure, that houses configuration
  options needed for bootstrap token based discovery.

- Introduce a FileDiscovery structure, that houses configuration options
  (currently only a single option) needed for KubeConfig based discovery.

- Introduce a Discovery structure, that houses common options (such as
  discovery timeout and TLS bootstrap token) as well as pointer to an instance
  of either BootstrapTokenDiscovery or FileDiscovery structures.

- Replace the old discovery related JoinConfiguration members with a single
  Discovery member.

This change is required in order to cleanup the code of unnecessary logic and
make the serialized JoinConfiguration more structured (and therefore, more
intuitive).
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>