Add support for setting env vars in deployment JSON

This will add support for defining environment variables that are passed into containers in deployment JSON. The main reason I made this is that when you are deploying eg. wordpress + mysql, then you have to set the MYSQL_PASS for the `mysql` image. Then on `mysql` run, this ENV var is passed into the mysql server.

Next use case is Docker registry, you have to pass couple env vars to configure it properly. While you can do it already using the `--env-file` option, there is no such option for the `deploy`.

```
{
    "IdPrefix": "simple-",
    "containers": [
        {
            "name": "test",
            "count": 1,
            "image": "openshift/busybox-http-app",
            "env": {
                "FOO": "bar",
                "MYSQL_PASS": "secret"
            }
        }
    ]
}
```

`$ gear deploy simple.json`

Then:

```
$ cat /var/lib/containers/env/contents/si/simple-test-1
FOO=bar
MYSQL_PASS=secret
```
This should not be a method on Environment description, it should be a method with the signature:

    func ToArray(map[string]string) EnvironmentVariables