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We have several Kinesis streams set up with event notifications that trigger lambdas.  When the LAMBDA_EXECUTOR is set to 'docker' and we send a large number or records to Kinesis, multiple Lambdas will execute, but currently they are failing to create with error:

```
Exception: Lambda process returned error status code: 1. Output:
time="2019-09-18T17:28:11Z" level=error msg="error getting events from daemon: net/http: request canceled" 
Error response from daemon: driver failed programming external connectivity on endpoint busy_sanderson (a78e45a41d0ce5b0d8bab6ff54344fc34fe2b4969f55bf568e753121abc8759e): Bind for 0.0.0.0:38763 failed: port is already allocated
```

I believe this is due to the new debug_java_port introduced with this PR https://github.com/localstack/localstack/pull/1523

With this change, you are setting the port in the constructor by using a method known to have a race condition (https://github.com/localstack/localstack/blob/master/localstack/utils/common.py#L354)
(https://gist.github.com/gabrielfalcao/20e567e188f588b65ba2)

This should be changed to ensure unique ports are being issued across all lambdas


""" Helper class to run a shell command in a background thread. """
    def __init__(self, cmd, params={}, outfile=None, env_vars={}, stdin=False,
            quiet=True, inherit_cwd=False, inherit_env=True):
        self.cmd = cmd
        self.process = None
        self.outfile = outfile or os.devnull
        self.stdin = stdin
        self.env_vars = env_vars
        self.inherit_cwd = inherit_cwd
        self.inherit_env = inherit_env
        FuncThread.__init__(self, self.run_cmd, params, quiet=quiet)
    def run_cmd(self, params):
        def convert_line(line):
            line = to_str(line or '')
            return '%s\r\n' % line.strip()
        def filter_line(line):
            """ Return True if this line should be filtered, i.e., not printed """
            return '(Press CTRL+C to quit)' in line
        try:
            self.process = run(self.cmd, asynchronous=True, stdin=self.stdin, outfile=self.outfile,
                env_vars=self.env_vars, inherit_cwd=self.inherit_cwd, inherit_env=self.inherit_env)
            if self.outfile:
                if self.outfile == subprocess.PIPE:
                    
                    streams = ((self.process.stdout, sys.stdout), (self.process.stderr, sys.stderr))
                    for instream, outstream in streams:
                        for line in iter(instream.readline, None):