test: give tmt vm 20G disk instead of 40G

Sometimes the deployed testing farm runner does not have big disk,
to avoid this issue, use smaller tmt vm disk instead

Signed-off-by: Xiaofeng Wang <[email protected]>

Author: henrywang

plans/integration.fmf

@@ -5,6 +5,7 @@ provision:
   how: virtual
   # Generated by `cargo xtask `
   image: file://./target/testvm/disk.qcow2
+  disk: 20
 summary: Basic smoke test
 execute:
   how: tmt