More flexibility for configuring jmeter scripts

[robertjahn]

Is your feature request related to a problem? Please describe.
My application runs on port 8080. If use the sample jmeter scripts, this Jenkinsfile that is part of Keptn will always pass in port 80. https://github.com/keptn/jenkins-service/blob/master/pipelines/Jenkinsfile.run_tests#L92

My application does not have a health page as defined in the keptn jenkinsfile
https://github.com/keptn/jenkins-service/blob/master/pipelines/Jenkinsfile.run_tests#L126

Describe the solution you'd like

1. have a way to specify the port to use
2. have a way to specify what script keptn pipelines call. Right now I have to make 3 scripts with specific names for example https://github.com/keptn/jenkins-service/blob/master/pipelines/Jenkinsfile.run_tests#L122
   Perhaps this is part of the service YAML file when it is onboarded.
3. have a way to specify healthcheck url, method, body

Describe alternatives you've considered
hardcode port and health page url in my jmeter scripts

[dirkwall]

related to #338: configuration for mapping of channels to Jenkins pipelines requested