Fixes #8273/BZ1138868: fix JS error on content hosts page.

Use track by in ng-repeat to fix javascript error on the
content hosts page when visiting multiple content hosts
sequentially.

http://projects.theforeman.org/issues/8273
https://bugzilla.redhat.com/show_bug.cgi?id=1138868
Merge pull request #4803 from waldenraines/8273

Fixes #8273/BZ1138868: fix JS error on content hosts page.