use plugin name as default artifact id

so we doesn't need both if they are equal

For example:

sonar::plugin { 'sonar-ldap-plugin' :
  version    => '0.1',
  notify     => Service['sonar'],
}
refresh service sonar after install a new plugin

so we doesn't do it ourself

For example:

sonar::plugin { 'sonar-ldap-plugin' :
  version    => '0.1',
}