Fix slave-user and slave-password with pt-table-checksum

[tobiaspal]

When using pt-table-checksum without --sync-to-master and the DSN as the master to sync all slaves, the --slave-user and --slave-password parameters are ignored.

Steps to reproduce:

1. Set up master-slave replication with percona-server-5.7
2. Create a user to be used as the slave-user with a random password:

   GRANT ALL PRIVILEGES ON *.* TO 'pt-table-checks' IDENTIFIED BY 'auW2phohsiey';
   

   This user will get replicated to the slaves.
3. Run pt-table-checksum to create the percona.checksums table:

   pt-table-checksum --slave-user=pt-table-checks --slave-password=auW2phohsiey
   

   This uses the slave-user parameter correctly and finishes successfully.
4. Run pt-table-sync on the master with the slave-user parameter to sync the slaves to the master:

   pt-table-sync --replicate=percona.checksums --dry-run localhost
   

   This fails with:

   Cannot connect to h=10.4.28.78
   

   Because it's not using the slave-user parameter to connect to the slave.

[CLAassistant]

All committers have signed the CLA.

[ChristianCiach]

Please merge this! I just wasted hours trying to find out why setting the "--slave-user" option has no effect.

[svetasmirnova]

Currently, you can provide a replica username and password as part of the DSN.

[svetasmirnova]

Other issues with the fix are:

• options take precedence even if user and password defined in the dsns table. This could cause issues if users have, say, 10 replicas with the same user/password pairs and two with different ones.
• you access tool options in the library code
• there is no test case.

There is also a similar PR at https://github.com/percona/percona-toolkit/pull/425

[svetasmirnova]

We will consider similar https://github.com/percona/percona-toolkit/pull/425 instead.