nodetool may try to boot with memory higher than what scylla leaves for other process to run

[slivne]

scylla may leave for the os (nodetool and everything else)

max(1.5GB, 0.07 * available_memory) * panic_factor of 1.

(according to scylla/seastar@641f07a37d4e40491465a2255d67eb1b451aae08)

this is especially bad for low memory machines and does not align with the memory allocation that nodetool is started with.

To fix this we should:

1. recommend/insist on enabling swap when we have less then 8GB free.
2. reduce the memory nodetool is started with (not clear why there is such a high memory requirement for nodetool to begin with) - I think we can change this to 1GB - the min value that can be set in the env.

nodetool uses the code in conf/cassandra-env.sh that has

   # set max heap size based on the following
    # max(min(1/2 ram, 1024MB), min(1/4 ram, 8GB))
    # calculate 1/2 ram and cap to 1024MB
    # calculate 1/4 ram and cap to 8192MB
    # pick the max
    half_system_memory_in_mb=`expr $system_memory_in_mb / 2`
    quarter_system_memory_in_mb=`expr $half_system_memory_in_mb / 2`
    if [ "$half_system_memory_in_mb" -gt "1024" ]
    then
        half_system_memory_in_mb="1024"
    fi
    if [ "$quarter_system_memory_in_mb" -gt "8192" ]
    then
        quarter_system_memory_in_mb="8192"
    fi
    if [ "$half_system_memory_in_mb" -gt "$quarter_system_memory_in_mb" ]
    then
        max_heap_size_in_mb="$half_system_memory_in_mb"
    else
        max_heap_size_in_mb="$quarter_system_memory_in_mb"
    fi
    MAX_HEAP_SIZE="${max_heap_size_in_mb}M"