Fix CUDF build/deploy

CUDF current requirement on Java 6 breaks the build or the deploy.

• a Maven job cannot use an older JDK than the running one => JDK 6 is not possible => JENKINS-28294

  [ERROR] BUILD FAILURE
  [INFO] ------------------------------------------------------------------------
  [INFO] Compilation failure
  
  org.eclipse.equinox.p2.cudf-master/src/main/java/org/eclipse/equinox/p2/cudf/solver/Projector.java:[57,16] error: type LexicoHelper does not take parameters

• Java 6 is not compliant with ECDHE ciphers => deploy does not work since the SSL security upgrade on Nexus:

  [INFO] [deploy:deploy {execution: default-deploy}]
  [INFO] Retrieving previous build number from vendor-snapshots
  [INFO] ------------------------------------------------------------------------
  [ERROR] BUILD ERROR
  [INFO] ------------------------------------------------------------------------
  [INFO] Error retrieving previous build number for artifact 'org.eclipse.equinox:org.eclipse.equinox.p2.cudf:jar': repository metadata for: 'snapshot org.eclipse.equinox:org.eclipse.equinox.p2.cudf:1.18-NX-SNAPSHOT' could not be retrieved from repository: vendor-snapshots due to an error: Error transferring file: java.lang.RuntimeException: Could not generate DH keypair
  

Possible solutions:

• update CUDF code to drop JDK 6 and use JDK 7 or 8
• make use of toolchains to actually use a different JDK during some Maven phases

See https://qa.nuxeo.org/jenkins/job/Vendor/job/org.eclipse.equinox.p2.cudf-master.old/ (Maven job) and https://qa.nuxeo.org/jenkins/job/Vendor/job/org.eclipse.equinox.p2.cudf-master/ (Freestyle job). If possible, the Maven job is preferred.