changed deprecated channel_label attribute

Since RHN Satellite 5.3 "channel_label" attribute is removed from the api (deprecated in RHN Satellite 5.2), it is replaced by "label" attribute.
https://access.redhat.com/site/documentation/en-US/Red_Hat_Network_Satellite/5.2/html/API_Overview/files/handlers/ChannelSoftwareHandler.html#listSystemChannels