cert-submitter: allow custom base domain.

[cpu]

Previously the base domain for all the test certificate subjects was a static string specific to Let's Encrypt. Allowing this base domain to be configured in the cert submitter config will help outside organizations use this tool with their own logs.

Resolves https://github.com/letsencrypt/ct-woodpecker/issues/105

[coveralls]

Pull Request Test Coverage Report for Build 857

• 20 of 20 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 69.243%

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Totals
Change from base Build 764:	0.4%
Covered Lines:	860
Relevant Lines:	1242

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💛 - Coveralls