Add GIT_BRANCH to AMI tags
Add GIT_BRANCH to AMI tags

This change adds the Jenkins `GIT_BRANCH` environment variable to Amazon
Machine Images after they're built.

azavea/nyc-trees,kdeloach/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees,maurizi/nyc-trees,kdeloach/nyc-trees,maurizi/nyc-trees,kdeloach/nyc-trees,azavea/nyc-trees,maurizi/nyc-trees,kdeloach/nyc-trees,maurizi/nyc-trees,azavea/nyc-trees,azavea/nyc-trees