frontend: use entryset in GWTRPCCommunicationProvider

Use entryset instead of keyset for more efficient map interation.
Solves findbug warning of type WMI_WRONG_MAP_ITERATOR

Change-Id: Ifaff47aaba96f6fb7a115af63c239b63077a4632
Signed-off-by: Alissa Bonas <abonas@redhat.com>