Support multiple roots per environment for external pillars S3 and Git

Issue #18355

In our Salt 2014.7 Master we encountered the issue that the pillar for each environment could only come from one root directory in the cache. To support a mix of S3 and Git external pillars, I had to put in the following with regards to how the `__opts__['pillar_roots'][environment]` lists were being populated.

I'm not comfortable modifying the other pillar modules since I'm not familiar with those systems, but maybe someone on the Saltstack project could apply a similar fix (if this change is acceptable).