qa: fix prometheus load generator floating point rounding
qa: fix prometheus load generator floating point rounding

Various platforms are formatting the prometheus.SOURCE.counterXXX
instance domain names with different floating point rounding. e.g.
for prometheus.sourceX.sample_counter0001, some platforms are
formating some of the "baz" instance names as e.g.

   inst [3 or "3 baz:2.0999999999999996"] value 1531848550

and others are formatting it (correctly) as e.g.

   inst [3 or "3 baz:2.1"] value 1531848550

Fix this by forcing a "{:.1f}" format specifier in the load generator
script for these instance names.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp