XD-2119,XD-2143,XD-2144

* Add 'spring-boot-starter-security' as a dependency
* Set up the admininstration and management endpoints as insecure by default
* Unit tests for SSL
* Unit tests for security enablement
Not sure what's the difference between the 2 (HTTPS is SSL). If 2 refers to customizing the client through the SSLContextBuilder, that is necessary in order to allow the trust chain to process the self-signed certificate we use for testing. It has no bearing on whether SSL is enabled or not on the server (which is what we try to establish). Probably some comments on that code could help.

I see what you're saying about testing with HTTP, though, but quite frankly, I don't think that those tests are really necessary - they simply produce a timeout (which also adds to the duration of the tests). While it is technically possible to have HTTP and HTTPS listening on the same port, it's quite a complicated undertaking and it's far beyond what Spring Boot itself and its embedded servers support - so once we've established that the target port listens to HTTPS, we can safely assume that HTTP is not supported. (We do that kind of test in reverse - testing HTTP first for SingleNodeApplicationWithDefaultSecurityTest, but in that case there's no timeout of the client,  and the failure is quite immediate, so it is less expensive).