(MODULES-1338) Allow mysql::db to import several files

A user might need to import several files on database creation.
Currently the module only allows the import of a single file.
This commit allows one to, from now on, import severals.

Before :
```puppet
  mysql::db { 'test' :
    sql => '/tmp/my_import1.sql',
  }
```
Now :
```puppet
  mysql::db { 'test' :
    sql => [
      '/tmp/my_import1.sql',
      '/tmp/my_import2.sql',
    ]
  }
```