Use single quotes in main `rpmbuild` command

This stops nesting of double quotes in in scenarios where the RPM is 
signed.
Don’t exit RPM creation when STDOUT =~ /failed/

There are non failure scenarios where failed may appear in warning 
message, for example:

  warning: File listed twice: /opt/chefdk/embedded/apps/chef/lib/chef/chef_fs/file_system/operation_failed_error.rb
Be sure to create and sign the RPM if `~/.rpmmacros` exists