[NEXUS-7336] Step 2: Get rid of security.xml in UTs and ITs

https://issues.sonatype.org/browse/NEXUS-7336
http://bamboo.s/browse/NX-OSSF377

To get rid of security.xml and security-configuration.xml:

* in UTs override "file" SecurityConfigurationSource/SecurityModelConfigurationSource with hardcoded test specific ones
* in ITs security is configured by calling into rest to setup the needed config