Make Spring Security SecurityContext available to method security interception.

The filter chain now registered, named "mySecurityFilterChain", doesn't clear SecurityContext to make it available to CXF, following service invocation and service invocation AOP interception.

The problem we faces comes from DOSGi not linking registered filter chain with CXF request processing, as explained here:
https://issues.apache.org/jira/browse/DOSGI-183
Instead, filter chain is executed before CXF request processing, which normally cleares the context, thus preventing it being accessible when method invocation is triggered.

To solve this issue, a customised version of org.springframework.security.web.context.SecurityContextPersistenceFilter is used
com.mytestcompany.sst.sec.SecurityContextPersistenceFilterSkipClearContext
which lets the user specify if context clearing should be skipped.

Using it with "skipClearContext" property set to true ensures that SecurityContext will be available in our environment, while DOSGI-183 is not fixed.