frontend: Removed "-" from "Data-Centers" in Err Msg

To be consistent with the UI's terminology, the error message "There are
no active Data-Centers in the system." was changed to "There are no
active Data Centers in the system.".

Change-Id: Ie1cdf14e67366b7007a64dc24e127623cfeee2c9
Bug-Url: https://bugzilla.redhat.com/1141794
Signed-off-by: Idan Shaby <ishaby@redhat.com>