burnin: Disk template option cleanup
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 <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

andir/ganeti,mbakke/ganeti,apyrgio/ganeti,dimara/ganeti,andir/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,grnet/snf-ganeti,mbakke/ganeti,andir/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,apyrgio/ganeti,grnet/snf-ganeti,ganeti/ganeti,yiannist/ganeti,onponomarev/ganeti,dimara/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,bitemyapp/ganeti,yiannist/ganeti,leshchevds/ganeti,leshchevds/ganeti,bitemyapp/ganeti