Module apt: cache_valid_time option

New feature for apt module: cache_valid_time option

Here's what it's about:
To ensure that you have no outdated apt cache most playbooks have update_cache set on nearly every invocation of apt in a playbook.
This can get unnecessarily slow if you run serveral playbooks in a row.
Because within one operation of running 5 playbooks in a row, you're updating the cache at least 5 times.

With the option cache_valid_time you can set a time in seconds for which the cache should be valid. Then the cache only gets updated if the last update on the current server is more than the specified time in seconds ago.