Add support of a default LCCI Image in the fusesoc() step

[oleg-nenashev]

In #6 @Nancy-Chauhan introduced a new fusesoc() step which requires to explicitly specify a version of the LibreCores CI to be used. It would be great to support a default version which is managed by Global properties in the library

@Library('librecoresci') _

pipeline {
    stages {
        stage('example-fusesoc-step') {
            fusesoc {
                image 'librecores/librecores-ci:0.5.0'
                library 'some_core', '/src'
        
                run('some_core') {
                    target 'fusesoc_target'
                }
        
                shell "echo 'Additional steps on shell'"
            }
        }
    }
}