distros: alias a-supported-distro.yaml to CentOS 7

Some jobs are distribution agnostic and it is meaningless to run them on
all supported distributions. For such jobs it is convient to require
than they run on a supported distribution but we don't care which. This
can be done by including a-supported-distro.yaml in the suite sub
directory instead of linking to distros/supported.

Signed-off-by: Loic Dachary <loic@dachary.org>

diff --git a/distros/supported/a-supported-distro.yaml b/distros/supported/a-supported-distro.yaml
new file mode 120000 (symlink)
index 0000000..f512e12
--- /dev/null
+++ b/distros/supported/a-supported-distro.yaml
@@ -0,0 +1 @@
+../all/centos_7.0.yaml
\ No newline at end of file