Adding support for cloud template selection

In this release, it is possible to create the project configurations more dynamically by selecting different cloud templates.
The file jcasc_config.yaml should now have a ##K8S_MGMT_JENKINS_CLOUD_TEMPLATES## placeholder:

  clouds:
    - kubernetes:
        name: "jenkins-build-slaves"
        serverUrl: ""
        serverCertificate: ##KUBERNETES_SERVER_CERTIFICATE##
        directConnection: false
        skipTlsVerify: true
        namespace: "##NAMESPACE##"
        jenkinsUrl: "http://##JENKINS_MASTER_DEPLOYMENT_NAME##:8080"
        maxRequestsPerHostStr: 64
        retentionTimeout: 5
        connectTimeout: 10
        readTimeout: 20
        templates:
##K8S_MGMT_JENKINS_CLOUD_TEMPLATES##

It is important, that the placeholder is at the beginning of the line.

If a folder called cloud-templates is existing in the templates folder, then all files in this directory will be shown as possible cloud-templates.
The user can then select which (none or multiple) sub-templates should be added to the main template.

These sub-templates must also start at the beginning of the line.
For an example have a look here: templates/cloud-templates/node.yaml