Fix certificate based authentication

[jandelgado]

Implements a workaround until streadway/amqp#121 is merged, so we can use cerfificate based authentication.

[coveralls]

Pull Request Test Coverage Report for Build 695310811

• 21 of 23 (91.3%) changed or added relevant lines in 5 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 93.476%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/rabtap/main.go	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
cmd/rabtap/main.go	2	83.07%

Totals
Change from base Build 376634362:	0.1%
Covered Lines:	2092
Relevant Lines:	2238

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💛 - Coveralls