Merge remote-tracking branch 'origin/develop_integration_2018_w15' into develop

Summary of changes:
- some work on better parallel processing of eNB
  try the eNB with options: --fepw --codingw
- small changes for multi-UE simulation and simulation accuracy
  (UE power levels)
- initial work for the UE to use TUN device instead of ue_ip.ko
  only done for oaisim, default bearer, compile with:
    ./build_oai --ue-nas-use-tun <rest of options>