grow-disk: wait until resync is completed

The patch adds a new ‘--no-wait-for-sync’ parameter to grow-disk similar
to the one in instance add, and changes the default to wait.

This is cleaner as at the moment when the command returns, we either
have a fully synced disk or there is an error.

This is a forward-port of rev 1183 on the 1.2 branch.

Reviewed-by: ultrotter