burnin: Disk template option cleanup

Burnin doesn't support all disk templates (specifically it doesn't
support “blockdev”). The code, however, used manually composed lists in
some places, constants.DISK_TEMPLATES in others, and an internal list of
supported templates in some.

This patch changes burnin to only use its internal list of supported
disk templates.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>