AP_Camera: add support for the user to specify which gimbal each camera is in

[khanasif786]

This adds a new parameter, CAMx_MNT_INST to allow users to specify which mount the camera is in. This is useful in numerous situations including allowing the correct mount angles to be logged at the moment a picture is taken.

[rmackay9]

I've put some comments in but this all looks very good to me, thanks!

[rmackay9]

I did a search through AP_Camera for, "mount->" and found that we've probably missed:

• AP_Camera_Backend::Write_CameraInfo() uses the ca,era instance when it should use the newly added method
• send_camera_information() and send_camera_settings() seem to send for all mounts which doesn't seem correct. Surely this should only be sending for one instance (this seems like my mistake, sorry!)

[rmackay9]

Looking good!

[peterbarker]

A couple of things to fix in stand-alone comments.

PeterB's issues address I think.

[rmackay9]

Merged, thanks!