Improvement/Refresh frequency for data sources

[buixor]

When scenarios/parsers are using data:source_url directives, it would be nice to be able to specify a "refresh" period (ie. "24h", "30d" etc.)

cscli hub upgrade (and similar sub-commands) would then check the file timestamp against a given refresh period.

if the refresh period is reached, it would then automatically re-download the resource, allowing users running cscli hub upgrade on a regular basis to ensure that they keep up-to-date resources.