Cleanup spot requests with price-too-low status

[NikolayS]

Since docker-machine rm --force ... wasn't helpful for removing Spot Requests with "price-too-low" status, we need to use AWS CLI to do that.

Additionally, in this PR

• createDockerMachine function doesn't use global vars anymore
• destroyDockerMachine function added
• cleanup function renamed to cleanupAndExit and moved to the top to allow interruptions (Ctrl-C ) at early stages
• summary items reordered
• some error messages from AWS pruned

How to check

Run any experiment on r4.xlarge instance (usually their spot prices are increased, sign of high demand):

echo "create table aaa as select i::int4 from generate_series(1, 1000000) _(i);" > ~/tmp/test.dump
bzip2 ~/tmp/test.dump

nancy run --workload-custom-sql 'select now();'   \
  --db-dump-path file://$(echo ~)/tmp/test.dump.bz2 --tmp-path $(echo ~)"/tmp"   \
  --run-on aws --aws-ec2-type r4.xlarge \
  --aws-keypair-name awskey --aws-ssh-key-path /home/nikolay/.ssh/awskey.pem

Something like that should appear after the 1st attempt to launch an instance:

Price .0700700 is too low for r4.xlarge instance. Getting the up-to-date value from the error message...
{
    "CancelledSpotInstanceRequests": [
        {
            "State": "cancelled",
            "SpotInstanceRequestId": "sir-icjg4brg"
        }
    ]
}

-- and in web console the corresponding spot request should be in "cancelled" state.