qa/suites/rbd: add random distro selection to librbd tests

Signed-off-by: Jason Dillaman <dillaman@redhat.com>

diff --git a/qa/suites/rbd/librbd/supported-random-distro$ b/qa/suites/rbd/librbd/supported-random-distro$
new file mode 120000 (symlink)
index 0000000..0862b44
--- /dev/null
+++ b/qa/suites/rbd/librbd/supported-random-distro$
@@ -0,0 +1 @@
+.qa/distros/supported-random-distro$
\ No newline at end of file