Ensure a stable content of the bash completion file

Currently, the order of commands in the bash completion file is
random, because the sub-commands are not sorted. This makes it harder
to investigate the differences in packaged Ganeti or in installed
Ganeti, since chunks in this file will have a random order.

To fix this, we sort the subcommands based on the first subcommand in
a given group ('first' also in sorted order); this results in a stable
contents of file, as tested by building it many times and checking for
differences.

The patch also does a few other minor changes to the file
(e.g. updating copyright years, etc.).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>