Implement behaviour for --clustergittrack-mode flag

[JoelSpeed]

Available options for the flag are:

• Disabled: Do not manage ClusterGitTracks
• IncludeNamespaced: Manage ClusterGitTracks and allow them to manage namespaced resources
• ExcludeNamespaced: Manage ClusterGitTracks but disallow them from managing namespaced resources

This behaviour should be achievable by manipulating which Watches are actually started based on flag configuration.

We will need to ensure that namespaced objects are ignored properly in the GitTrack controller when the value of this flag is ExcludeNamespaced.