Use the m-lab/go/host package to parse -hostname flag

[nkinkade]

This provides a consistent interface for processing a hostname, and allows us to handle special cases in the host package instead of in this code. For example, the host package knows about managed instance group random suffixes on instance/node names, and can handle that gracefully for uuid-annotator.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 289

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 283:	0.0%
Covered Lines:	760
Relevant Lines:	760

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💛 - Coveralls