Description of problem:
[RFE] Do not allow negative numbers for Activation key Usage limit

Version-Release number of selected component (if applicable):
* apr-util-ldap-1.3.9-3.el6_0.1.x86_64
* candlepin-0.9.19-1.el6_5.noarch
* candlepin-scl-1-5.el6_4.noarch
* candlepin-scl-quartz-2.1.5-5.el6_4.noarch
* candlepin-scl-rhino-1.7R3-1.el6_4.noarch
* candlepin-scl-runtime-1-5.el6_4.noarch
* candlepin-selinux-0.9.19-1.el6_5.noarch
* candlepin-tomcat6-0.9.19-1.el6_5.noarch
* elasticsearch-0.90.10-4.el6sat.noarch
* foreman-1.6.0.17-1.el6sat.noarch
* foreman-compute-1.6.0.17-1.el6sat.noarch
* foreman-gce-1.6.0.17-1.el6sat.noarch
* foreman-libvirt-1.6.0.17-1.el6sat.noarch
* foreman-ovirt-1.6.0.17-1.el6sat.noarch
* foreman-postgresql-1.6.0.17-1.el6sat.noarch
* foreman-proxy-1.6.0.7-1.el6sat.noarch
* foreman-selinux-1.6.0-4.el6sat.noarch
* foreman-vmware-1.6.0.17-1.el6sat.noarch
* katello-1.5.0-26.el6sat.noarch
* katello-ca-1.0-1.noarch
* katello-certs-tools-1.5.5-1.el6sat.noarch
* katello-installer-0.0.48-1.el6sat.noarch
* openldap-2.4.23-32.el6_4.1.x86_64
* pulp-katello-0.3-3.el6sat.noarch
* pulp-nodes-common-2.4.0-0.20.beta.el6sat.noarch
* pulp-nodes-parent-2.4.0-0.20.beta.el6sat.noarch
* pulp-puppet-plugins-2.4.0-0.20.beta.el6sat.noarch
* pulp-puppet-tools-2.4.0-0.20.beta.el6sat.noarch
* pulp-rpm-plugins-2.4.0-0.20.beta.el6sat.noarch
* pulp-selinux-2.4.0-0.20.beta.el6sat.noarch
* pulp-server-2.4.0-0.20.beta.el6sat.noarch
* python-ldap-2.3.10-1.el6.x86_64
* ruby193-rubygem-net-ldap-0.3.1-3.el6sat.noarch
* ruby193-rubygem-runcible-1.1.0-2.el6sat.noarch

How reproducible:
Always

Steps to Reproduce:
# hammer activation-key update --name='actkey' --organization-id=1 --environment-id=2 --content-view='cv1' --usage-limit=-19100
Activation key updated

Actual results:
-19100 is accepted as a valid value for usage limit

Expected results:
Do not accept negative numbers for usage limit

Additional info:
I understand we use -1 to represent "unlimited" internally, but we should not accept any negative numbers for Usage limit from the user.