Fix bash autocompletion of "rally task sla_check"

Character "_" is used as delimiter of commands and
their subcommands stored in bash array as keys,
i.e. in form "task_sla_check". Current code splits
it into an array based on delimiter "_" which would
return array with three values - task, sla and check.
The patch takes first value from the beginning to
first "_" as command and the rest as its subcommand.

Change-Id: I7d8c9b1eeb7c28ef24b0f2f366c6a18930282be5
Closes-bug: 1401180

etc/rally.bash_completion

@@ -50,8 +50,9 @@ _rally()
 
 
     for OPT in ${!OPTS[*]} ; do
-        CMDSUB=(${OPT//_/ })
-        SUBCOMMANDS[${CMDSUB[0]}]+="${CMDSUB[1]} "
+        CMD=${OPT%%_*}
+        CMDSUB=${OPT#*_}
+        SUBCOMMANDS[${CMD}]+="${CMDSUB} "
     done
 
     COMMANDS="${!SUBCOMMANDS[*]}"