Travis: Add MySQL 8.0 test enviornment

[julienschmidt]

Description

• added a test environment using a docker container with MySQL 8.0
• set local-infile=1 via command args (defaults to 0 in MySQL 8)
• changed the wait script to grep the output, since the mysql client does not support all the required auth plugins (such as MySQL 8.0's default auth plugin).
• removed the sleep command and poll the status more aggressively instead, potentially reducing the wait time

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[arvenil]

@julienschmidt A suggestion. Don't use mysql client from travis host. Use the one in docker image. This way mysql client 8 will talk to mysql server 8 and you won't need any additional hacks.
https://github.com/percona/qan-agent/blob/master/.travis.yml#L73

docker-compose exec mysql mysql -e 'SELECT 1'

[julienschmidt]

Thanks for the hint! Do you mind sending a pull request?

[arvenil]

I can do it later, right now fighting with MySQL 8 support ;)