/**
 * returns all available bitbucket projects
 * @return  list of projects
 */
def java.util.List getBitbucketProjects() {
    echo("[INFO] Get all bitbucket projects")
    String url = "${bitbucketHost}/projects/"
    String response = this.queryBitbucket(url)
    def list = new JsonSlurper().parseText(response)    
    def projects = []
    list.values.each { 
        projects.add(it.key)
    }
    return projects
}

...
        [$class: 'ChoiceParameter', 
            choiceType: 'PT_SINGLE_SELECT', 
            description: 'Select Bitbucket project from Dropdown List', 
            filterLength: 1, 
            filterable: true,
            name: 'project', 
            script: [
                $class: 'GroovyScript', 
                fallbackScript: [
                    classpath: [], 
                    sandbox: false, 
                    script: 
                    """
                       return [ "Could not get bitbucket projects" ]
                    """
                    ], 
                script: [
                    classpath: [], 
                    sandbox: false, 
                    script: 
                    """
                        return this.getBitbucketProjects()
                    """
                ]
            ]
        ], 
...