Add support for jaeger collector and agent endpoints in config-tracing

[ian-mi]

Fixes #1076 and #1077

When the jaeger backend is specified, exactly one of the collector and agent endpoints should be specified. Note that this does not add support for agent endpoints using node ip + port.

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-pkg-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
tracing/config/tracing.go	73.5%	73.8%	0.3
tracing/opencensus.go	50.0%	39.0%	-11.0

[knative-prow-robot]

@ian-mi: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-knative-pkg-go-coverage	3a05778	link	/test pull-knative-pkg-go-coverage

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[knative-prow-robot]

@ian-mi: PR needs rebase.

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