Part of DEV-1083 better handling of INI file for mysql client

[moseshll]

• Write credentials to tempfile based on ENV
• Remove config/mysql_defaults_extra.ini
• Requires hathifiles-database secret/ENV with connection string broken into individual variables

[aelkiss]

Given our current understanding that dumping with the mysql/mariadb command line client is significantly faster than any approach we could take with Ruby, I think this is fine -- it puts the knowledge all in one place about database credentials, whereas other approaches would need the credentials duplicated.

I don't think we need the gsub to replace whitespace with spaces, but worth double checking, but removing that doesn't need to block merging.

[moseshll]

The gsub on the SQL heredoc is redundant since the gsub applied to the system("mysql ...") command takes care of it. The top-level gsub is needed -- system gets mad otherwise. I'm getting rid of the redundant one.