OA-40: Add property to control starting of the module

[corneliouzbett]

Issue: https://openmrs.atlassian.net/browse/OA-40

This PR adds property oauth2.enabled to control starting of the module. It defaults to true to ensure backward compatibility if the property is not set.

[wluyima]

I have left 2 comments

[ibacher]

Two high-level comments:

1. This is... very clever, and I get the "cleanliness" of simply stopping and unloading the module. This is maybe something we could consider adding in core as part of a shouldStart callback in the activator?
2. Stopping a module is potentially a very costly operation. This is because it can trigger a context refresh on trying to unload the module. All that is meant to suggest that this is probably fine in this limited case, but this would be a very bad pattern to adopt globally (hence my suggestion of building the functionality into core, since we can, at the very least, batch all the modules that were loaded but shouldn't start into a single context refresh).

[wluyima]

Left some nitpick comments, but this is approved to be merged

[corneliouzbett]

Two high-level comments:

1. This is... very clever, and I get the "cleanliness" of simply stopping and unloading the module. This is maybe something we could consider adding in core as part of a shouldStart callback in the activator?
2. Stopping a module is potentially a very costly operation. This is because it can trigger a context refresh on trying to unload the module. All that is meant to suggest that this is probably fine in this limited case, but this would be a very bad pattern to adopt globally (hence my suggestion of building the functionality into core, since we can, at the very least, batch all the modules that were loaded but shouldn't start into a single context refresh).

Thank you @ibacher for the insightful comments. Your comments makes sense to me. I will create a TRUNK ticket to further evaluate the possibility of incorporating this functionality into the core.