RSCBC-147: Fix various issues with running unit tests against a cluster_run-based cluster

[Peter-Searby]

This doesn't fix all of the issues faced when using cluster_run (some of which may or may not just be caused by flakiness in the tests), but does cover the following:

• Some tests depend on a fixed mgmt port, which can't be met by a cluster_run. To fix this http bootstrapping support is added, so that a http:// connection string can be used to make the tests compatible with a cluster_run instance
• Any tests depending on fixed ports for other services are switched to using ns_server's /_p/<service>/ proxy API, so that the mgmt port can be used for all such requests (note, this is not needed for any tests depending on any auto-discovery of ports)
• The get_terse_cluster_config function was incorrectly using a streaming API, causing it to hang when used for bootstrapping, this has been switched to the correct API

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