Generic function to print data for gnt-xxx info

The function produces a human-readable output, which is also a valid YAML
file, from an intermediate data structure. This will be used by "gnt-xxx
info" commands. The output of "gnt-xxx info" was almost YAML-compliant, and
only minor adjustments are needed to make it fully compliant.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>