Setup global context with timeout

[Pokom]

To safe guard cloudcost-exporter from initiating too many external requests that do not get cancelled, there should be a uniform way of passing a Context that has a CancelFunc defined with a sane default. This should start at exporter.go as a parameter with a 1m default set and then passed to each provider that gets created. The provider can then generate a context with a timeout defined on each Collect call and pass this on to the collectors Collect methods.

To achieve this, the collector interface needs to be refactored to have the Collect method accept a context as well.

[Pokom]

@Pokom and @logyball will follow up with a design.