Introduce sechub-server-core gradle subproject

[de-jcup]

Problem

We have failing doc test cases #144 and #166 because we got following situation:

sechub-server --> builds sechub-server-x.y.z (but its a bootjar!)
+src/main/java/.../InfoService.java

sechub-doc --> gradle dependency to sechub-server(but as normal jar.. 
+test/main/java/.../SomeTest.java which uses InfoService.java

Gradle subproject sechub-server is designed to be the composite part for server and was not intended to be used as dependency but as a boot jar only.

But we got some services and rest controllers inside only for server reason - e.g. server alive and server version info etc.

Solution

We will create a new gradle subproject sechub-server-core were all stuff being not only configuration but code, resources etc. will be settled. And so it will be accessible in standard gradle way.

[de-jcup]

We need still some refactorings:

• packages inside sechub-server-core must contain core
• some configurations parts shall move back to sechub-server
• sechub-server package names should be com.daimler.sechub.server instead of com.daimler.sechub