Program cloudwatchlogs-relay listens for rsyslogd messages over unix socket
and sends messages to CloudWatch logs.

To send all logged messages on a host to CloudWatch Logs, configure rsyslogd
by creating file /etc/rsyslog.d/cloudwatch.conf with the following content:

    $ModLoad omuxsock
    $OMUxSockSocket /var/run/cloudwatch
    *.* :omuxsock:

Restart rsyslogd, and run this program like this:

    cloudwatchlogs-relay -socket=/var/run/cloudwatch \
        -group=/groupname/rsyslogd \
        -stream='instance/${INSTANCE_ID}'

This will create log group "/groupname/rsyslogd", and inside it a log stream
"instance/${INSTANCE_ID}", where ${INSTANCE_ID} would be replaced with EC2
instance ID. Supported placeholders for replacement are: ${HOSTNAME},
${INSTANCE_ID}, ${RANDOM} — for instance ID, hostname, and random string. Be
careful to make sure placeholders are not expanded by the shell (use single
quotes).

Program requires the following IAM permissions:

    {
        "Version": "2012-10-17",
        "Statement": [
            {
                "Effect": "Allow",
                "Action": [
                    "logs:CreateLogGroup",
                    "logs:CreateLogStream",
                    "logs:PutLogEvents"
                ],
                "Resource": "*"
            }
        ]
    }

Note that while program tries to remove socket on clean shutdown, it is
still possible that leftover socket file may be left, preventing new
instance from starting.

It relies on AWS SDK and expects to find credentials/region settings in
usual places (IAM role, environment, configuration files).