instead of printing the result of the "validate" subcommand in a
free-style plain text, let's print it using JSON. for two reasons:
1. it is simpler to consume the output with other tools and tests.
2. more consistent with other commands.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16105