Document consistency model
Fix several issues in docs/consistency-model.md

- Clarify that some operations don't care about the dirty bit

- Use 'apply' rather than 'deploy'

- Clarify failure mode and idempotency of 'project_apply'

- Fix description of 'node_{attach,detach}_network'

- Document limitation of 'headnode_delete'

- Needlessly reindent paragraph for 'headnode_power_{on,off}'
Use 'frozen' terminology
Rename headnode_power_{on,off} to headnode_{start,stop}
Remove confusing clause in consistency model doc
Replace 'connect' with 'attach'
Merge pull request #175 from gsilvis/model-doc

Document consistency model