Burnin OpGrowDisk

Burnin OpGrowDisk

With this patch both the os and the swap disk are grown during
burnin. You can pass an increase size of 0 to skip this operation.

and:

burnin: don't try to grow diskless instances

    When burnin is run on a diskless instance, it fails when trying
    GrowDisk, because a non-existant disk cannot be grown. This patch
    disables the test for that disk template.

This is a forward-port of revisions 1181 and 1606 on the 1.2 branch.

Original author: ultrotter
Reviewed-by: ultrotter