Add env to projects

[JrGoodle]

Allow custom environment variables to be specified in clowder.yml for the clowder forall command.

See: https://gerrit.googlesource.com/git-repo/+/master/docs/manifest-format.md#element-annotation

Possible names: env, environment

projects:
  myproject:
    name: 'myname/myproject'
    env:
      VARIABLE_NAME: 'variable_value'