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
---|--------------------------------------|-----------------
fixes #5601, #5603 - host-collections: add support for update and add/remove-content-host

This commits add support for the following commands:

host-collection update
host-collection add-content-host
host-collection remove-content-host
Merge pull request #162 from bbuckingham/issue-5602

fixes #5602 - host-collections: add command to list content-hosts
Merge pull request #163 from bbuckingham/issue-5601

fixes #5601, #5603 - host-collections: add support for update & add/remove-content-host