Description of problem: The `hammer hostgroup <create|update>` subcommands take organizations or organization_ids, and the command is successful if only one organization is provided; however, when providing a comma-separated list, the following error is displayed:

Could not create the hostgroup:
  Error: organization found more than once


Version-Release number of selected component (if applicable):
6.1.8, upstream

How reproducible:
Always in 6.1.8 and 1.12.0-develop.

Steps to Reproduce:
1. Create an additional organization:
hammer organization create --name "org3"
2. Find the org's ID:
hammer organization list
3. Try to create a host group with 2 or more organizations:
hammer hostgroup create --content-view 'cv1' --organizations "Default Organization","org3" --name 'hg3'

Actual results:
Could not create the hostgroup:
  Error: organization found more than once

Expected results:
Hostgroup created

Additional info:
Related (PR submitted): https://bugzilla.redhat.com/show_bug.cgi?id=1309107