Fixes add-apt-keys in dpkg element

This commit addresses the issue described in bug 1768354 when using the
apt-sources element and adding a key to a custom repo, subsequent deb
package installs fail due no update of the repo before package install

Change-Id: I968b3422fab2fb2305426d49215391d8ba7499df
Closes-Bug: 1768354

diskimage_builder/elements/dpkg/pre-install.d/02-add-apt-keys

@@ -33,3 +33,5 @@ for KEY in ${KEY_DIRECTORY}/*; do
 
     apt-key add ${KEY}
 done
+
+apt-get -y update