Correct MIME Type for YAML files

The currently specified MIME types "*/yaml", "*/x-yaml" are not generally found on any popular web server. See http://www.iana.org/assignments/media-types/media-types.xhtml . Consequentially, the MIME type returned for a YAML file hosted via a common web server is text/plain . This change shouldn't affect XML files served since they do have a globally accepted MIME type and will be served with that in the header.

Facing issues with being able to serve such a file off a shared service. Seeing...

ERROR ExceptionCatchingResourceModelSource: [ResourceModelSource: 1.url (URL Source), project: XYZ] 
com.dtolabs.rundeck.core.resources.ResourceModelSourceException: Error requesting URL Resource Model Source: http://f.q.d.n/xyz.yaml: Response content type is not supported: text/plain

Caused by: com.dtolabs.rundeck.core.resources.format.UnsupportedFormatException: No provider available to parse MIME type: text/plain
        at com.dtolabs.rundeck.core.resources.format.ResourceFormatParserService.getParserForMIMEType(ResourceFormatParserService.java:193)
        at com.dtolabs.rundeck.core.resources.URLResourceModelSource.getNodes(URLResourceModelSource.java:309)

...in the logs.