add new key to get vmware's value "ready"

According to https://www.zabbix.com/documentation/2.4/manual/config/items/itemtypes/simple_checks/vmware_keys zabbix key for vmware's "ready" parameter is missing now.

The official description for this is:
"Percentage of time that the virtual machine was ready, but could not get scheduled to run on the physical CPU. CPU ready time is dependent on the number of virtual machines on the host and their CPU loads."

Ref: http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.wssdk.apiref.doc%2Fcpu_counters.html

Please add support of new item key

Added specification: https://www.zabbix.org/wiki/Docs/specs/ZBXNEXT-2619