modified list_entities to gather ALL entities via pagination

Problem
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Currently when using the cloudmonitorclient class, a request for the list of entities returns only the first page. This was un-intuitive, as it was expected to provide all entities.

Approach
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By modifying BaseManager to save it's 'next_marker' on a paginated result, it is able to provide it to CloudMonitorClient which can then use it to request ALL entities via the necessary number of paginated api requests.
The returned list is the concatenated result from each api page result.

Risks
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BaseManager: should be low risk. only capturing the marker if it is indeed set
CloudMonitorClient: Will loop API requests until the api gives either no next_marker or a None. Could be until end of time if the API keeps giving next markers