Add support for managing Cloud DNS zones

[stephen-soltesz]

This change updates the dnsx.Manager by adding two new methods for: RegisterZone and RegisterZoneSplit. The combination of these methods will make it possible to register a new organization and create the Cloud DNS zone and resource records needed by autojoined nodes from that organization.

A zone split works in two steps:

• Create a new zone for the organization, which automatically includes NS records, e.g. foo.sandbox.measurement-lab.org
• Add a new NS record to the parent project zone, which is the "split" and allows DNS resolvers to recursively lookup a name, e.g. sandbox.measurement-lab.org contains the NS records for foo.sandbox.measurement-lab.org

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 10796257601

Details

• 42 of 42 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 98.803%

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Totals
Change from base Build 10784136990:	0.05%
Covered Lines:	1073
Relevant Lines:	1086

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💛 - Coveralls

[cristinaleonr]

Reviewable status: complete! 1 of 1 approvals obtained