We have already defined all the constant etcd versions in the
centralized place api/version/version.go. So we should replace all
the versions with the centralized definitions.
Problem with old code was that during downgrade only members with
downgrade target version were allowed to join. This is unrealistic as
it doesn't handle any members to disconnect/rejoin.
Storage version should follow cluster version. During upgrades this
should be immidiate as storage version can be always upgraded as storage
is backward compatible. During downgrades it will be delayed and will
require time for incompatible changes to be snapshotted.
As storage version change can happen long after cluster is running, we
need to add a step during bootstrap to validate if loaded data can be
understood by migrator.
