- PXC#460: wsrep_sst_auth don't work in Percona-XtraDB-Cluster-56-5.6.25-25.12.1.el7

Semantics:
---------

* Generally end-user will create a separate user with needed
privileges for
    performing DONOR action.
* This user credentials are specified using wsrep_sst_auth.

* Along with this user there could be other user(s) created on the
server
    that sysadmin may use for normal or other operations
* Credentials for these user(s) can be specified in same
cluster/server
    cnf file as part of [client] section

When cluster act as DONOR and if wsrep_sst_auth is provided then it
should
strictly use it for performing SST based action.

What if end-user has same credentials for performing both SST action
and
normal admin work ?
* Then end-user can simply specify these credentials as part of
[client]
    section in cnf file and skip providing wsrep_sst_auth.

Issue:
-----

MySQL client user/password parsing preference order is as follows:
* command line (through --user/--password)
* cnf file
* MYSQL_PWD enviornment variable.

Recent change tried passing sst user password through MYSQL_PWD
(and user though --user command line param as before).

On the system where-in admin had another user for performing non-SST
actions,
credentials for such user were present in cnf file under [client]
section.

Due to mysql client preference order, SST user name was used (as it
was
passed through command line) but password of other user (meant for
non-SST)
action was being used as it was passed through cnf file.

Password passed through MYSQL_PWD was completely ignored causing
user-name/password mismatch.

Solution:
---------

* If user has specified credentials for SST then pass them through
command
    line so that they are used in priority.

(There could be security concern on passing things through command
line but
   when I tried passing user-name and password through command line to
mysql
   client and then did ps I saw this
   ./bin/mysql --user=sstuser --password=x xxxxxxxx -S /tmp/n1.sock
so seems like password is not shown)

scripts/wsrep_sst_xtrabackup-v2.sh

@@ -689,9 +689,8 @@ then
            INNOEXTRA+=" --user=$WSREP_SST_OPT_USER"
         fi
 
-        if [ -n "$WSREP_SST_OPT_PSWD" ]; then
-#           INNOEXTRA+=" --password=$WSREP_SST_OPT_PSWD"
-           export MYSQL_PWD="$WSREP_SST_OPT_PSWD"
+        if [ -n "${WSREP_SST_OPT_PSWD:-}" ]; then
+           INNOEXTRA+=" --password=$WSREP_SST_OPT_PSWD"
         else
            # Empty password, used for testing, debugging etc.
            INNOEXTRA+=" --password="

scripts/wsrep_sst_xtrabackup.sh

@@ -442,9 +442,8 @@ then
            INNOEXTRA+=" --user=$WSREP_SST_OPT_USER"
         fi
 
-        if [ -n "$WSREP_SST_OPT_PSWD"  ]; then
-#           INNOEXTRA+=" --password=$WSREP_SST_OPT_PSWD"
-           export MYSQL_PWD="$WSREP_SST_OPT_PSWD"
+        if [ -n "${WSREP_SST_OPT_PSWD:-}" ]; then
+           INNOEXTRA+=" --password=$WSREP_SST_OPT_PSWD"
         else
            # Empty password, used for testing, debugging etc.
            INNOEXTRA+=" --password="