Add the ability to provide pre-generated certs

[alexmasi]

Currently self-signed is the only cert type provided, this PR adds another option to the topo proto but does not yet add support to node impls to use the new field.

A follow up PR will use the field.

[coveralls]

Pull Request Test Coverage Report for Build 9768506569

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 65.198%

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Files with Coverage Reduction	New Missed Lines	%
topo/node/arista/arista.go	1	71.34%

Totals
Change from base Build 9765987846:	-0.01%
Covered Lines:	4646
Relevant Lines:	7126

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💛 - Coveralls

[chrisy]

This has been top of mind for me, so thanks for tackling it. Generating certs is the slowest part of topology creation in many cases!