libs/utils/executor: check CGroups name for proper formatting
libs/utils/executor: check CGroups name for proper formatting

All CGroups which can be configured for a workload have top be expressed
relative to the root control group.

This patch generates a run-time error if the CGroup specified for a
target configuration is starting with a "/".

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

credp/lisa,joelagnel/lisa,ARM-software/lisa,joelagnel/lisa,arnoldlu/lisa,ARM-software/lisa,bjackman/lisa,ARM-software/lisa,mdigiorgio/lisa,mdigiorgio/lisa,JaviMerino/lisa,ARM-software/lisa,credp/lisa,credp/lisa,arnoldlu/lisa,credp/lisa,bjackman/lisa