Add release key for installing packages from ceph.com

In commit bb773e7285 the key for
verifying release packages from ceph.com was removed when support for
the ceph package mirrors was added. However, it is still needed for
non-gate case of fetching packages from ceph.com, so this adds it
back in that case.

Closes-Bug: #1578494

Change-Id: I18f8927f42710e9365c246c55f26f53403965ee1

devstack/lib/ceph

@@ -621,6 +621,10 @@ function install_ceph {
             # packages from ceph.com.
 
             sudo apt-add-repository "$APT_REPOSITORY_ENTRY"
+
+            # install the release key for ceph.com package authentication
+            wget -q -O- 'https://download.ceph.com/keys/release.asc' \
+                | sudo apt-key add -
         fi
 
         if [ "$ENABLE_CEPH_RGW" = "True" ]; then