fixes #5602 - host-collections: add command to list content-hosts

Added a list command to show the list of content-hosts in a
host-collection. E.g.

hammer> host-collection content-hosts --id 1
---|--------------------------------------|-----------------
ID | UUID                                 | NAME
---|--------------------------------------|-----------------
9  | 054925b5-2046-494d-afe8-8b56dc5a5596 | sat6client.devel
---|--------------------------------------|-----------------
Merge pull request #162 from bbuckingham/issue-5602

fixes #5602 - host-collections: add command to list content-hosts