(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',
    ]
  }
```
for one, i'm only now notiicing `/usr/bin/mysql` - that's not really cool for anything that's not linux.
i'm not happy with the use of `ls` & xargs here.

is there a less convoluted way to do that?
```puppet
command => "cat ${sql_inputs} | mysql ${dbname}"
```