Support dict keys in patterns

[lkubb]

What does this PR do?

Adds support for using dict keys in patterns.

Example pillar:

roles:
  web:
    some: config
  mail:
    other: stuff

Example pattern:

vault:
  policies:
    assign:
      - salt_minion
      - 'salt_role_{pillar[roles]}'

Example output:

- salt_minion
- salt_role_web
- salt_role_mail

What issues does this PR fix or reference?

Fixes: #26

Previous Behavior

Renders the whole dictionary as a single policy

New Behavior

Renders dictionary keys like lists

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes