scylla_swap_setup: add --swap-size-bytes
scylla_swap_setup: add --swap-size-bytes

Currently, --swap-size does not able to specify exact file size because
the option takes parameter only in GB.
To fix the limitation, let's add --swpa-size-bytes to specify swap size
in bytes.
We need this to implement preallocate swapfile while building IaaS
image.

see scylladb/scylla-machine-image#285

Closes #9971

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla