add certificate checking functionality and tests

Add the ability to print the certificate's subject and expiration
date/time.  Add information on using these new functions to the
troubleshooting docs.  Add unit tests.
11 jobs for master in 4 minutes and 20 seconds (queued for 2 seconds)
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  Build
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build:debian:jessie

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build:el7

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build:tarball

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  Test
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docs

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syntax

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