Burnin: test set-mem

Add new --maxmem-size and --minmem-size options.
The old --mem-size option allows setting both at the same time.
If --minmem-size is lower than --maxmem-size execute a runtime memory
change on all instances, bringing them to their minimum memory size.

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