CI Tests: Simplify the test plan syntax, keeping the semantics.

plans/ci.fmf

@@ -6,9 +6,8 @@ discover:
     directory: tests
     filter: 'tier: 1'
 prepare:
-    how: install
-    exclude:
-        - glibc-headers-x86
-        - glibc-headers-s390
+    - name: prepare
+      how: shell
+      script: ./plans/prepare.sh
 execute:
-    how: beakerlib
+    how: tmt

plans/ci/prepare/prepare.fmf

@@ -1,9 +0,0 @@
-summary: Setup task
-prepare:
-    script: ./plans/ci/prepare/prepare.sh
-recommend:
-- bind-utils
-- avahi-tools
-- systemtap
-- kernel-devel
-- kernel-debuginfo