Made var 'env' available in template rendering and allow subdirectories

[paulrhodes]

To allow for more granular control of the pillar data a couple of changes have been made:

• the fabric env 'env' is now passed in to the jinja template
• when rendering additional templates, cotton now understands the dotted naming scheme and the associated subdirectory structure.

This allows for a top.sls file which looks something like this:

base:
  '*':
    - base.common
    - {{env.release_stage}}.common

This makes it easier to use standard hierarchical definitions of pillars and the deep merging functionality it provides to minimise changes at the leaves.