Add more possible values for update frequency

[webcompas]

Use Case

Currently $apt::update only accepts "always", "daily", "weekly" or "reluctantly" as a valid "frequency" value. Depending on Puppet's "runinterval" setting it would be better to have more and flexible options.

Proposed solution

I'd suggest to add at least two additional valid values:

• "hourly": 3600 seconds since last update
• Integer: Number of seconds since last update