integration test: end to end server testing

[msimerson]

Fixes #1725

Changes proposed in this pull request:

• add server integration test: starts up a Haraka server and delivers an email using nodemailer
• replace TLS cert/key stubs with self-signed certs, so they're usable
• replace test/config/dhparam key with 2048 key (so nodemailer doesn't kvetch)
• remove logger.colorize code for legacy node version
• trailing whitespace in comment removed
• tests/plugins: after setting for tests, unset process.env.HARAKA and avoid side effects that interfere with other tests (like my new one)
• queue/discard: add ENV that permits discarding via ENV variable, b/c integration tests don’t have access to the connection/transaction object
• outbound: set queue_dir to tests/test-queue when env.HARAKA unset. It’s a more sane default than ‘/path/to/haraka/undefined/queue’
• tls.ini: add missing .pem extension to dhparams comment
• plugins: export config, so tests can overload it

Checklist:

• tests updated

[codecov-io]

Codecov Report

Merging #1791 into master will increase coverage by 10.03%.

@@             Coverage Diff             @@
##           master    #1791       +/-   ##
===========================================
+ Coverage   36.12%   46.16%   +10.03%     
===========================================
  Files          22       22               
  Lines        5793     5805       +12     
  Branches     1453     1457        +4     
===========================================
+ Hits         2093     2680      +587     
+ Misses       3700     3125      -575

Impacted Files	Coverage Δ
logger.js	81.1% <ø> (+3.75%)	✅
config.js	90.27% <100%> (+0.42%)	✅
outbound.js	11.45% <30.76%> (+0.18%)	✅
server.js	33.7% <54.16%> (+14.04%)	✅
plugins.js	73.52% <75%> (+23.2%)	✅
configfile.js	70% <ø> (+0.29%)	✅
messagestream.js	73.49% <ø> (+4.41%)	✅
... and 2 more

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