Gitman ignores groups on nested gitman install runs

Nobody has claimed this yet.

We have just decided to switch from an inhouse developed, almost identical tool, to gitman. But I've stumbled across what seems to be a bug, during my initial testing:

Given a gitman.yml file that looks like this:

```yaml
location: .gitman
sources:
# Production sources
- name: tf-module-production
link: tf-module
repo: git@githost:tf-module.git
rev: 1.2.0
# Dev sources
- name: tf-module-dev
link: tf-module
repo: git@githost:tf-module.git
rev: master
groups:
- name: production
members:
- tf-module-production
- name: dev
members:
- tf-module-dev
```
I expect that I end up with a directory looking like this, if I do a `gitman install dev`:

```
├── .gitman
├── .gitman.yml
└── tf-module -> .gitman/tf-module-dev
```

And if I do a `gitman install production`:

```
├── .gitman
├── .gitman.yml
└── tf-module -> .gitman/tf-module-production
```
This works - so far so good.

But if my tf-module contains a gitman.yml file with a similar structure and I run `gitman install production`, gitman actually installs all the sources, linking for each source, and it seems like I end up with the dev sources (so I end up with the master branches). I guess that gitman doesn't honour the groupname in the subsequent gitman install calls...?

Reproduce the nested gitman.yml setup and run `gitman install production` from the outer project, then inspect the `gitman install` command's handling of group names during recursive installs. Done means the nested project installs only the sources in the selected production group rather than all sources or the dev revision.

Written by the indexing model from the issue text.