FIX: HIGH PRIORITY multipath blacklist (rackerlabs#787)

Our current regexp pattern ^sda[0-9]* and ^sdb[0-9]* blacklists
sda1, sda2, sda3, sda4 which is what we want. Unfortunately,
it also blocks /dev/sda[a-x], /dev/sdb[a-x] which is NOT what
we want Guarav's Rally testing revealed that with a high number of
multipath devices, they naming convention starts to add the additional
alphabet letter as it scales up.

The correction: "^sd[a-b]?" per
https://www.suse.com/support/kb/doc/?id=000016563

Author: dwith

ansible/playbooks/templates/genestack-multipath.conf

@@ -9,8 +9,7 @@ blacklist {
     devnode    "scini*"
     devnode    "^rbd[0-9]*"
     devnode    "^nbd[0-9]*"
-    devnode    "^sda[0-9]*"
-    devnode    "^sdb[0-9]*"
+    devnode    "sd[a-b]$"
     devnode    "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
     devnode    "^(td|ha)d[a-z]"
 }