closes #320: implement findCollections command

[ivansenic]

What this PR does:
Implements the findCollections command.. Lists only jsonapi collections in a namespace. Example similar to findNamespaces:

$ curl -X 'POST' \
>   'http://localhost:8080/v1/cycling' \
>   -H 'accept: application/json' \
>   -H 'X-Cassandra-Token: 90e68c40-c254-43cb-87d2-91c9003de546' \
>   -H 'Content-Type: application/json' \
>   -d '{
>   "findCollections": {}
> }' | jq .

{
  "status": {
    "existingCollections": [
      "events"
    ]
  }
}

Which issue(s) this PR fixes:
Fixes #320

[vkarpov15]

LGTM overall, one trivial suggestion

[ivansenic]

@johnsmartco we would also need spec for this new command as well..

[ivansenic]

I think adding this class was not needed.. I can simply use the SchemaManager in the ops.. I see no future need for extending this one, thus I might remove it..

[ivansenic]

Done.

[jeffreyscarpenter]

Converted to draft as suggested by @maheshrajamani

[ivansenic]

@jeffreyscarpenter @maheshrajamani This was ready to review a long time ago, I don't understand why is this moved to draft and still not reviewed?

[maheshrajamani]

@jeffreyscarpenter @maheshrajamani This was ready to review a long time ago, I don't understand why is this moved to draft and still not reviewed?

@ivansenic I have suggested to move it to draft because of your comment to "add test for this class" and also I did not find any unit test cases for those classes.

[maheshrajamani]

LGTM