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.

[LaurenceJJones]

We added support to official sceanarios, in short when upgrade is ran it will send a HEAD request to the hub CDN to get a last modified timestamp if the file on disk is long out of date it will then proceed to download said resources again.

The user cannot control the pull timings, as this is purely done through http headers