Add bash-completion rules

This is a not-complete bash completion file for ganeti commands (gnt-*)
and the burnin tool. It is based on previous work by Minghua Ye
<yeminghua@google.com> for Ganeti 1.1, which wasn't used because the
lack of ssconf keys (which allow easy inspection by the shell of the
existing nodes and instances) made it too slow.

The file works as expected, however I realized that our custom (like
comma-separated, or a=b:c,e:f) options are not very nice for
auto-completion. There are a few FIXMEs in the source for that.

The file is not installed at make install time, but it should be put in
the correct place by packages.

Reviewed-by: imsnah