Adding custom data maps to InfluxDB from pipelines

[nocono]

extending what has been provided with #7.

With the CustomDataMapPointGenerator data generated in a pipeline can easily be written into Influx.
The measurement name is be defined as key of the respective map which itself contains another map with key-value pairs containing measurement data.

Example (also available as comment in InfluxDbPublisher):

def myDataMap1 = [:]
def myDataMap2 = [:]
def myCustomDataMap = [:]
myDataMap1["myMap1Key1"] = 11 //first value of first map
myDataMap1["myMap1Key2"] = 12 //second value of first map
myDataMap2["myMap2Key1"] = 21 //first value of second map
myDataMap2["myMap2Key2"] = 22 //second value of second map
myCustomDataMap["series1"] = myDataMap1
myCustomDataMap["series2"] = myDataMap2
step([$class: 'InfluxDbPublisher', target: myTarget, customPrefix: 'myPrefix', customDataMap: myCustomDataMap])

This pull request also provides unit tests for both customDataPointGenerator as well as customDataMapPointGenerator.