Enhancements to xtrabackup backup provider

[fraenki]

This PR will bring the xtrabackup provider up to par with the mysqldump provider and adds one other enhancement:

• Add support for existing parameters $backupuser and $backuppassword
• Automatically create backup user/grants
• Add support for the databases parameter to provide a list of databases to back up
• Add support for optional arguments to innobackupex by adding the new parameter $optional_args

This makes it possible to specify additional arguments to be passed to innobackupex:

mysql::server::backup:
  provider: 'xtrabackup'
  user: 'backupusr'
  password: 'secret'
  databases:
    - 'foo'
    - 'bar'
  optional_args:
    - '--no-lock'
    - '--parallel=2'

[bmjen]

thanks @fraenki can you take a look at the spec test failures? Also, the new options need to be documented in the README.

[fraenki]

@bmjen I've fixed the unit test and added documentation for the only new parameter optional_args (all other options aren't new, but were not supported previously by the xtrabackup provider).

[bmjen]

Thanks @fraenki !