Add blacklisted/hidden OS support in LUDiagnoseOS

This changes the behaviour of LUDiagnoseOS significantly.

The addition of hidden/blacklisted OSes would mean that each user-facing
client would have to filter intentionally such OSes from display, which
is not a good choice. Rather, the patch makes LUDiagnoseOS not return
any hidden or blacklisted OSes unless the hidden or respectively the
blacklisted status is requested.

While unconventional, this makes `gnt-instance reinstall --select-os`
work as intended without any changes; similar for gnt-os list. gnt-os
diagnose/gnt-os info are changed to query for, and display the new
fields.

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