Remove usage of context manager in cgroups.py
Remove usage of context manager in cgroups.py

Context managers are not supported before Python 2.7. Removes the
use of the 'with' clause in cgroups.py because this code is
executed on Centos 6 packaging boxes with an older version of
Python.

Change-Id: Ic6bcf161086f671ec2010df16f9bb23534c57697
Reviewed-on: http://gerrit.cloudera.org:8080/385
Reviewed-by: Martin Grund <2546548d516fa64c411fde8242af35f3f80ad31f@cloudera.com>
Tested-by: Matthew Jacobs <df2fa7c39d141dc5dc5589fed74180e757ed6c9a@cloudera.com>

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