DRILL-143: Support CGROUPs resource management
Introduces the DRILLBIT_CGROUP option in drill-env.sh.
The startup script checks if the specified CGroup (ver 2) is available and tries to apply it to the launched Drillbit JVM.
This would benefit not just Drill-on-YARN usecases, but any setup that would like CGroups for enforcement of (cpu) resources management.
(Also introduced SYS_CGROUP_DIR to account for possible non default locations of CGroup).
e.g when Drillbit is configured to use `drillcpu` cgroup
```
[root@maprlabs ~]# /opt/mapr/drill/apache-drill-1.14.0-SNAPSHOT/bin/drillbit.sh restart
Stopping drillbit
..
Starting drillbit, logging to /var/log/drill/drillbit.out
WARN: Drillbit's CPU resource usage will be managed under the CGroup : drillcpu (up to 4.00 cores allowed)
```
e.g. Non-existent CGroup `droolcpu` is used
```
[root@kk127 ~]# /opt/mapr/drill/apache-drill-1.14.0-SNAPSHOT/bin/drillbit.sh restart
Stopping drillbit
..
Starting drillbit, logging to /var/log/drill/drillbit.out
ERROR: cgroup droolcpu does not found. Ensure that daemon is running and cgroup exists
```
closes #1200
