Remove brackets from IPv6 address in mysql cfgfile

stack.sh creates a user-specific configuration file ~/.my.cnf for mysql.
If devstack is installed with SERVICE_IP_VERSION=6 option in local.conf,
the IPv6 host address was stored in the ~/.my.cnf file with square
brackets. However mysql does not use bracketing for IPv6 addresses,
resulting in 'Unknown MySQL server host' error when 'mysql' command is
run. With this patch IPv6 host address is written to ~/.my.cnf without
brackets.

Closes-Bug: #1516776
Change-Id: I27a7be8c75cf6b09b4a75dc4c9d09cd36bc5ac81

Author: Johan Pas

lib/databases/mysql

@@ -143,7 +143,7 @@ MYSQL_PRESEED
 [client]
 user=$DATABASE_USER
 password=$DATABASE_PASSWORD
-host=$DATABASE_HOST
+host=$MYSQL_HOST
 EOF
         chmod 0600 $HOME/.my.cnf
     fi