tmt: ask for bigger disks and get "df -h" output after testing

The tests are currently running out of disk space inside the
testing-farm environment. This commit should help debug this
by adding `df -h` output after the test is run.

It also asks for 200GB disk space (which is probably way more
than we need). Note that the docs say that the size is 250GB
by default for things running inside the RH ranch so this might
be redunant [1].

[1] https://docs.testing-farm.io/Testing%20Farm/0.1/test-request.html#hardware

plans/all.fmf

@@ -5,6 +5,8 @@ provision:
   hardware:
     virtualization:
       is-supported: true
+    disk:
+      - size: ">= 200GB"
 prepare:
   how: install
   package:
@@ -17,3 +19,6 @@ prepare:
 execute:
   how: tmt
   script: pytest -s -vv --force-aws-upload
+finish:
+  how: shell
+  script: df -h