Json config file values must be url encoded
It was reported that when pulp is configured through katello-installer and user specifies a value for proxy-user or proxy-password parameter, it's propagated to
/etc/pulp/server/plugins.conf.d/yum_importer.json (and other similar files) without being url encoded. I suspect that the need of using url encoded values is actually a pulp issue, wherever the json is used, pulp should encode it according to context. An example of failing value is "passw?rd" which needs to be converted to "passw%24rd". While we could likely make katello-installer encode the value in this file, if you later change this, it would break again.
For additional information, see https://bugzilla.redhat.com/show_bug.cgi?id=1180637. Just note that it combines actually two issues (shell escaping or installer arguments is separate issue).