[Fix #4493] Support for Required Environment Variables

[eliat123]

This is my first PR.

I have added support for required environment variables - issue #4493.
Required environment variables are regular environment variables ending with '?'.

Whenever a required environment variable is written in docker-compose configuration: for example:

volumes:
  - ${MISSING?}:/app/code
environment:
  - MISSING?

we get an error message such as:

ERROR: Invalid interpolation format for "volumes" option in service "web": "${MISSING?}:/app/code" caused by required environment variable "MISSING" not set

and

ERROR: environment variable "MISSING?" is required

Please note:

• All tests are passing and added new tests for this feature.
• Did not find a version to place this in the changelog.md
• Documentation is on a different repo, I will add it once I get approval for this PR.
• Tried to follow @rsynnest suggestions to implement this feature.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" git@github.com:eliat123/compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[eliat123]

I have recommitted my work as previous jenkins build was stuck, andrewhsu@docker.com has asked me to re-trigger a commit to initialize the build process.

[eliat123]

FYI, I have accidentally removed my changes from my fork and this caused this issue to close, I have re-merged the fix on #4778