Add process exit code to MachineProcess & process_died event

[voievodin]

What does this PR do?

Adds exit code to the process struct & process_died event.
Example of REST API response when getting a dead process GET /process/1

{
  "pid": 1,
  "name": "test",
  "commandLine": "echo hello",
  "type": "test",
  "alive": false,
  "nativePid": 1833,
  "exitCode": 0
}

Example of process_died jsonrpc event:

{
  "jsonrpc": "2.0",
  "method": "process_died",
  "params": {
    "time": "2017-05-26T10:19:09.056360416+03:00",
    "pid": 2,
    "nativePid": 2524,
    "name": "print",
    "commandLine": "printf \"1\n2\n3\"",
    "exitCode": 0
  }
}

What issues does this PR fix or reference?

Resolves #5147

Changelog

Added exit code to the response of process REST/WS API and to the process_died event body

[codenvy-ci]

Build # 2679 - FAILED

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