Add a info subcommand to gnt-job

Currently, it is hard to examine a job in detail; the output of ‘gnt-job
list’ is not easy to parse.

The patch adds a ‘gnt-job info’ command that is (vaguely) similar to
‘gnt-instance info’ in that it shows in a somewhat easy to understand
format the details of a job.

The result formatter is the most complicated part, since the results are
not standardized; the code attempts to format nicely the most common
result types (as taken from a random job list), via a generic algorithm.

Reviewed-by: imsnah