Use hostname from .conbench config in cpp adapter

[alistaire47]

Per Elena, the cpp microbenchmarks were sending the wrong hostname, e.g. this one should be a generic AWS name instead of the actual AWS machine name.

This uses self.conbench.machine_info attribute for the cpp adapter because that grabs the hostname from the .conbench config file, if populated, which is how we're populating machine_info.name on some runners.

Long-term, we need to think about how and where we want to store this type of metadata, given we've moved a lot of other things from .conbench to env vars. Given I'll need to reconfigure this whole thing anyway to run the adapter directly from arrow-benchmarks-ci, I'm ok with this short-term patch (which is really what most of this file is), as I'll have a better view then of all the other data like this we'll need to wrangle and hopefully have a more useful opinion about how to handle it.

[coveralls]

Pull Request Test Coverage Report for Build 3833623361

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.222%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
benchmarks/cpp_micro_benchmarks.py	0	1	0.0%
<!--	Total:	0	1	0.0%	-->

Totals
Change from base Build 3739399116:	0.0%
Covered Lines:	1488
Relevant Lines:	1706

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💛 - Coveralls