Support for environment variables substitution

[surajssd]

Now user can put environment variables in docker-compose file and it will be read by kompose from environment

Fixes #56

[surajssd]

The code in this PR is from https://github.com/docker/libcompose/blob/master/docker/project.go#L25

So I needed some discussion around this PR, so I checked the code in kompose here, it is similar to code in libcompose here. So I was hoping we can use the function NewProject from libcompose which is under docker package not under project package.

So the downside of using docker package's NewProject is it returns project.APIProject and I would like to have a method in project.APIProject that returns internal data structure of type *Project and we don't need to repeat the code. So I will be working with libcompose to get this done. If that works out this PR will change!

[ngtuna]

@surajssd I see you are changing libcompose code in /vendor. Is it possible to just work on kompose code base and keep libcompose as a dependency ?

[surajssd]

@ngtuna I did godep save to get latest code from libcompose. Is it not the right way to do it?

[ngtuna]

@surajssd Ah okay you changed libcompose revision to HEAD. OK let me check it.

[surajssd]

@ngtuna @kadel How can I convert a APIProject interface object type to Project type? Is there a way to do it?

[ngtuna]

@surajssd The reason I didn't use NewProject function because I don't want kompose to be constrained by libcompose. Basically we only need its parsing function in order to parse docker compose file into komposeObject, nothing more. Otherwise, we have to follow libcompose's constraints such as it's required to have a docker-compose.yml file handy at the same folder.

Did you check whether your changes in cli/app/app.go still works with the current libcompose revision at HEAD master? If yes then I suggest to keep the vendoring stable and just apply your changes in app.go. I can't check it now but will do tomorrow morning.

[surajssd]

@ngtuna I checked with the vendored libcompose available I am not able to build, because I need https://github.com/skippbox/kompose/pull/67/files#diff-41d801ef80f1858d5e8e9695667e4dafR749 and it is not in the vendored libcompose

[ngtuna]

Checked! Thanks @surajssd . Please go ahead to merge upstream.

[surajssd]

@ngtuna thank you :)