(#11966) Only invoke apt-get update once.

(#11966) Only invoke apt-get update once.

Move apt-get update exec to a seperate class to minimize the number of
apt-get updates invoked by configuration changes.

* remove apt_update exec resource in apt class.
* remove apt-get-${name} in defines.
* apt::source notify Exec['apt update'].
* Remove dependency to Exec['apt_update'].
* fix rspec-puppet tests.

Conflicts:

	manifests/source.pp