Small fix for starting pulsar in webless mode

[gregvonkuster]

When pulsar is configured with --mq, running this command:

pulsar --mode webless

will throw this exception:

FileNotFoundError: [Errno 2] No such file or directory: '~/pulsar/server.ini.sample'

This occurs because ini_path is initialized to ~/pulsar/server.ini.sample here https://github.com/galaxyproject/pulsar/blob/master/pulsar/main.py#L254, and the sample config file is not created by pulsar-config when passing the --mq option.