Fix influxdb "field type conflict" error

Fix influxdb "field type conflict" error

JIRA: YARDSTICK-1457

Field types must be consistent across points in a measurement, otherwise
influxdb rejects newer points with "field type conflict" error.

This error was faced when "theor_max_throughput" field, meant to be
assigned as a float from "TxThroughput" sample, was instead assigned as
an int from initial value of theor_max_thruput variable. This especially
occured for subsequent packet sizes, when theor_max_thruput variable was
reset to its default int value.

This fix changes the default value of theor_max_thruput variable to 0.0,
and also makes sure that the default value for "Actual_throughput" field
is consistently a float.

Change-Id: If8d4f1297094709fe9657ab3e2b8adaad09815b2
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>
(cherry picked from commit 2f4418798a3cf245f863a617410403f226062ac8)