Change Desktop Background for VMs

[bluesentinelsec]

Our VM setup scripts need to be able to change the desktop backgrounds.

The desired end state is that the user logs in and sees a unique background for the attacker VM, and a unique background for the target VM.

We don't have the final desktop images yet, so for now, I'll suggest you make two simple desktops with black background and overlay text.

The text should say:

MITRE ATT&CK Defender
ATT&CK Adversary Emulation
Hostname: < attacker > or < target >

[garunagiri]

Addressed in #9