frontend: Removed "-" from "Data-Centers" in UIConstants

To be consistent with the UI's terminology, the phrase "Data-Centers"
was replaced with "Data Centers" in two UI constants.

Change-Id: I39411ebc018bdc11e4692ebc9c2909ef399f40a7
Bug-Url: https://bugzilla.redhat.com/1141794
Signed-off-by: Idan Shaby <ishaby@redhat.com>