Revert CephFS test image to Bionic Beaver

There's an issue with CephFS in Focal Fossa (Ubuntu
20.04 LTS) [1]. While this is being investigated,
lets revert to using Ubuntu Bionic Beaver (18.04 LTS)
for the test VMs.

[1] https://tracker.ceph.com/issues/47236
Change-Id: I221b4a48b2f66327c83a78514b471cae51e20ea8
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>
1 file changed