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
Actually, that is tested in org.springframework.xd.dirt.security.SingleNodeApplicationWithDefaultSecurityTest.testSslNotEnabledByDefaultForAdminEndpoints and org.springframework.xd.dirt.security.SingleNodeApplicationWithDefaultSecurityTest.testSslNotEnabledByDefaultForManagementEndpoints, respectively.