Add entry for "zonegroup" in context of Ceph

[daobrien]

From IRC conversation: "Several zones can be grouped into a zonegroup, previously known as a region." trying to work out if "zonegroup" is a formal name, proper noun, or whatever as part of the product. Ordinarily it should be two words Answer: according to http://docs.ceph.com/ Zonegroup: A zonegroup consists of multiple zones...

Recommend adding this as "zonegroup: one word when used to refer to the concept of a group of zones in Ceph. Capitalize at the beginning of a sentence." or something similar.