Support for network agnostic builds

Hi - Is there a way that the same yml can be used on different networks where the git repo links are different?
We have two networks where the git repos are mirrored but their links are different. So I was wondering if we could use environment variables in `sources` like:

```
location: repos

sources:
- repo: ssh://git@"${GIT_BASE}"/project/myrepo1.git
name: myrepo1
rev: "${PROJ_BRANCH:-develop}"

- repo: ssh://git@"${GIT_BASE}"/project/myrepo2.git
name: myrepo2
rev: "${PROJ_BRANCH:-develop}"
```

Where GIT_BASE and PROJ_BRANCH can be set before running gitman install.

Start at the `gitman install` entry point and trace how `sources` entries, repository URLs, and `rev` values are parsed. Check whether configuration already supports environment substitution; done means one YAML works with network-specific GIT_BASE and PROJ_BRANCH values, with appropriate coverage for the supported behavior.

Written by the indexing model from the issue text.