Ceilometer endpoint doesn't need last slash

The last slash is unnecessary, it only causes redundant slash when
generate api uri, which already treated as a bug in Ceilometer.

Change-Id: I758d76e8edab6e818989bac8e05eb17c6e373546
Closes-Bug: #1269496
backport: havana
ref: https://review.openstack.org/#/c/63279/
(cherry picked from commit ab15bf187971be7f3db2dec1be3bc1d37f72d6a1)