Allow remoteType to be overridden at the Version level

[carrolp]

If it becomes desirable to allow having a mix of multiple remoteTypes (e.g. both github and gitlab) Versions under the same Channel, this change will allow it:

• remoteType can still be set on the Channel, but becomes optional
• remoteType can be set on each Version
• The remoteType from the Version is used if set, else the one from the Channel if set. Neither set, no remote type is passed to the cluster (which will probably result in an error as it won't know how to handle it)