Remove restriction on a mix of security group names and IDs
Remove restriction on a mix of security group names and IDs

- Also more clearly describes precedence of security groups fields
- Removes content duplication

[#119844019](https://www.pivotaltracker.com/story/show/119844019)

Signed-off-by: Corey Innis <c6de6a297c7ba72017894924528d266f693d3f31@pivotal.io>

cloudfoundry/docs-bosh,voelzmo/docs-bosh