Fixes #6078, #6300, BZ1105175, BZ1097054: fix invalid limit fields.

This commit changes the way we handle limited fields such as Content Host
number of systems and Activation Key usage limit.  Instead of representing
"unlimited" as -1 we now represent it as a boolean on the object.

http://projects.theforeman.org/issues/6078
http://projects.theforeman.org/issues/6300
https://bugzilla.redhat.com/show_bug.cgi?id=1105175
https://bugzilla.redhat.com/show_bug.cgi?id=1097054

Goes with https://github.com/Katello/hammer-cli-katello/pull/199
Do folks like the current column naming?  i.e.
- unlimited & usage_limit (for activation keys)
- unlimited & max_content_hosts (for host collections)

Would users know that the fields are related?  If not, should we be more verbose?  e.g.
- usage_unlimited & usage_limit (for activation keys)
- unlimited_content_hosts & max_content_hosts (for host collections)

I'm not personally a huge fan of either, hence the questions... :)