Don't evaluate role list if hosts are defined.
Don't evaluate role list if hosts are defined.

This prevents Capistrano failing when a task has undefined
roles specified but hosts have been given.

e.g.

task :install_mysql, :roles => :db do
  # install mysql...
end

cap install_mysql HOSTS=some.new.host

dstrctrng/alpha_omega,dstrctrng/alpha_omega