sysctl can fail to set a value even if it returns an exit status 0. More
details: https://bugzilla.redhat.com/show_bug.cgi?id=1264080. Because of
this in case of an invalid value or a read-only file system, sysctl
module would return OK, even though it didn't set anything. To be sure
that sysctl correctly applied the changes we also need to check the
output of stderr.
