tests: explicitly use centos 7.3 in distros/supported

Signed-off-by: Nathan Cutler <ncutler@suse.com>

diff --git a/qa/distros/supported/centos.yaml b/qa/distros/supported/centos.yaml
deleted file mode 120000 (symlink)
index 32753e4..0000000
--- a/qa/distros/supported/centos.yaml
+++ /dev/null
@@ -1 +0,0 @@
-../all/centos.yaml
\ No newline at end of file

diff --git a/qa/distros/supported/centos_7.3.yaml b/qa/distros/supported/centos_7.3.yaml
new file mode 120000 (symlink)
index 0000000..34d81be
--- /dev/null
+++ b/qa/distros/supported/centos_7.3.yaml
@@ -0,0 +1 @@
+../all/centos_7.3.yaml
\ No newline at end of file