Cache Installation of uDocker in S3

[gmolto]

Upon the first execution, uDocker automatically retrieves an installation package from https://owncloud.indigo-datacloud.eu/index.php/s/lR6CsSP5HfEc5uZ/download.

In order to speed up the container deployment process, this package should be cached into S3.

[alpegon]

The package is now uploaded via the 'init' command, so it's automatically deployed with the function and don't need to be downloaded every time the function is called.
See pull request #18

[gmolto]

The downside of this approach is that we lose the ability to use the inline code editor in the AWS Lambda console but it pays off at this stage.