! akka-cluster: metrics collector improvements - resolve #16121

1) no need for "manually" - let jenkins do the work
https://github.com/akka/akka/pull/16354/files#diff-32787a354393f5aa1c24af6d86e9a736R18
BTW: 
1.1) should ```akka.test.sigar``` be ```true``` by default?
1.2) should have 2 test passes: both ```false``` and ```true```?

2) verified both a) and b); looks "nice" to me, except sigar logging
https://github.com/hyperic/sigar/blob/master/bindings/java/src/org/hyperic/sigar/SigarLog.java
which should be silenced/configured during sigar agent invocation #16355