Adding custom data to InfluxDB from pipelines & fixing issue with customPrefix

[nocono]

This pull request is intended to add some flexibility to the plugin.

Data within pipelines can be simply created.
With the CustomDataPointGenerator this can easily be written into Influx as "jenkins_custom_data"
In a pipeline script data is passed as Map to customData as follows:

def myDataMap = [:]
myDataMap['myFirstKey'] = 'myValue'
myDataMap['mySecondKey'] = 1234
step([$class: 'InfluxDbPublisher', target: myTarget, customPrefix: 'myPrefix', customData:myDataMap])

The second part of the pull request is to fix an issue with customPrefix. Without @DataBoundSetter the customPrefix is not taken into consideration from a pipeline script.