api docs: use array type instead of 'tags'

We had incorrectly implemented the swagger 1.2 spec with regard to
arguments that accept arrays. Rather than using a "tags" key, we
should have been using an "items" key. Change the canvas swagger
json generator to use "items": { type: "<type>" }

Test Plan:
- JSON output from `rake doc:api` should not have any "tags" keys
- JSON output should have, e.g. "items": { "type": "string" } in
  APIs whose arguments are array types and previously were
  described as "tags"

Fixes SIS-647

Change-Id: I262b1ced1d659332b430890fb5a2cf6e5c8909ee
Reviewed-on: https://gerrit.instructure.com/44246
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ken Romney <kromney@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>