Fix Fog::Compute::Google::Servers#all paging

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If there are more instances than will fit in a page a next_page_token is returned. This has to be passed in to subsequent calls in order to retrieve the full list of servers.

Note: I think this method could use some refactoring but I wanted to model this after the way Snapshots#all was fixed for consistency and change as little as possible for a targeted bug fix.

Fixes #607

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I'm curious on the maintainers' points of view on this but options to this method include :max_results and conceptually it isn't clear if this is intended to mean "never return more than this number of results" or specifically meant to be passed directly to the list_servers and list_aggregated_servers methods.

These options were added here 9891535#diff-bb76744c2fc47cf4c93fd72cfe6563ceb8d1421a342dd36a583fac4e49178142R7-R8 in what looks like simply adding explicit kwargs for each of the options that list_servers and list_aggregated_servers take.

I could see a user wanting to limit both the total results and the size of each page so maybe an additional option is needed?

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cc @Temikus

[Temikus]

LGTM

Regarding semantics - that particular commit was during a massive quick-and-dirty rewrite when google-cloud lib broke compatibility, so I'm in favor of a refactor to whatever is more user-friendly.