ICSHWI-7104 Added ioc_realtime and per-IOC realtime option

Enabling the realtime option adds the --realtime command line option to iocsh.bash to support realtime scheduling. Added CAP_SYS_NICE to the enabled capabilities to let the process change its priority.

Also, changed docker_testing to use a standard Centos/7 based VM instead of docker as the realtime test seems to fail under docker