Cloud based VNC access: Testing on Linux

The original investigation was done as part of https://github.com/ISISComputingGroup/IBEX/issues/5507. This ticket is to test the Linux VNC server and client, which was not carried out as part of 5507 because of the unavailability of instrument analysis machines.

## acceptance criteria

- In the linux VNC server
- instrument scientists able to grant and revoke vnc access to user (to limit to duration of their experiment)
- instrument scientist able to choose read only or read/write access
- adding users and pc's to relevant VNC groups understood to make user management easier
- able to access with correct permissions from a Linux client