Incorrect fallback for sidekiq_user for Systemd

[attenzione]

Systemd :sidekiq_user fallback to ssh user is incorrect, because there is no global fetch(:run_as):

  def sidekiq_user
    fetch(:sidekiq_user, fetch(:run_as))
  end

Upstart plugin has the same method and it's correct:

  def sidekiq_user(role = nil)
    if role.nil?
      fetch(:sidekiq_user)
    else
      properties = role.properties
      properties.fetch(:sidekiq_user) || # local property for sidekiq only
        fetch(:sidekiq_user) ||
        properties.fetch(:run_as) || # global property across multiple capistrano gems
        role.user
    end
  end

These two plugins contains more methods (sidekiq_user, switch_user, etc) that should be moved to common file.