(PE-3401) (REVIEW ONLY) Expose webserver setting override function

This commit includes a new function on the webserver service,
`override-webserver-settings!`, which can be used to override any of the
settings in the webserver section of the service's Trapperkeeper config
file prior to the webserver being started.  This commit has a proposed
implementation and tests but, assuming it is workable, would need to be
supplemented with some documentation in the README.md on how use the new
function.

This commit also removes the `certificate-whitelist` setting from the
jetty-config.md file as it was found to not be used anywhere in the
webserver implementation.
(PE-3401) Moved override-webserver-settings! tests to new namespace

This commit moves the override-webserver-settings! tests from the
jetty9-service namespace to a jetty9-service-override-webserver-settings
namespace in preparation for a larger breakup of the jetty9-service
tests to come on a later PR.  No functional changes are present in the
source code as part of the migration.
(PE-3401) Log overridden settings

This commit adds a log/info call with the name of each webserver setting
being overridden through a call to override-webserver-settings!
(PE-3483) Use CA lib for SSL, instead of KS
(PE-3401) Added docs for override-webserver-settings! to README.md

Added a section for the `override-webserver-settings!` function to the
README.md.
(PE-3401) Addressed README.md and test feedback from pull request

This commit corrects a reference to the `overrides` parameter in the
README.md.

This commit also contains several test updates:

- Removed unnecessary Service1 service protocol.
- Removed unnecessary str function invocations from one test for
  strings that didn't involve concatenation.
- Adjusted testing string on an SSL certificate settings override test
  to better describe the purpose of the test.
- Replaced `second-override-throws?` variable with
  `second-override-result` variable in second override attempt test to
  better indicate what the variable value could hold.
- Removed unused require statement.
Merge pull request #17 from cprice404/maint/master/PE-3483-use-ca-lib-for-ssl

(PE-3483) Use CA lib for SSL, instead of KS
Merge pull request #16 from camlow325/feature/master/PE-3401-expose-jetty-ssl-config

(PE-3401) Expose webserver setting override function
Update CHANGELOG for 0.5.0 release
Merge pull request #18 from cprice404/maint/master/changelog-for-0.5

Update CHANGELOG for 0.5.0 release
lein-release plugin: preparing 0.5.0 release
lein-release plugin: bumped version from 0.5.0 to 0.5.1-SNAPSHOT for next development cycle
Remove clj-http dependency

Conflicts:
	test/clj/puppetlabs/trapperkeeper/services/webserver/jetty9_service_test.clj
Reorganize service tests

This commit doesn't contain any code changes; it just reorganizes
the tests a bit so that you can run them with a little more
granularity.  In particular, it separates all of the SSL tests
that are intended to result in a successful HTTPS request into
one deftest, and all of the ones that expect a failed HTTPS
request into a separate deftest.
Fix tests based on PR review

This commit fixes some tests that weren't initializing certs correctly.
It also updates to the new version of the certificate-authority
library, which allows us to execute tests where we are setting
a ca cert on the client without setting a client cert/key.