'Gets the property value
:param prop: property name (file)
:param pwd: cgroup directory
:return: [] values or None when FAILED'

def get_property(self, prop, pwd=None):

    if (pwd is None):
        pwd = self.root
    if isinstance(pwd, int):
        pwd = self.cgroups[pwd]
    try:
        ret = [_[:(-1)] for _ in open(os.path.join(pwd, prop), 'r')]
        if ret:
            return ret
        else:
            return ['']
    except Exception as inst:
        raise error.TestError(('cg.get_property():    %s' % inst))
