webadmin - Fix report error msg on selecting entities across DCs

When selecting multiple Hosts/VMs from different
DCs and opening a report Via Webadmin. An incorrect
error message is displayed

"Please select reports from the same data center"

The message has been rephrased

"Please select entities from the same data center"

Change-Id: Ied2cbfd83aca1add35d7575fb117ed255385a0dc
Bug-Url: https://bugzilla.redhat.com/885983
Signed-off-by: Ravi Nori <rnori@redhat.com>