(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',
    ]
  }
```
@igalic I though I tried the cat solution and it didn't work. I tried back now and it seems to work, so yes I agree to go with that one

```bash
[root@monitor sql]# cat query1.sql 
CREATE DATABASE test_1;
USE test_1;
SELECT User FROM mysql.user;
[root@monitor sql]# cat query2.sql 
CREATE DATABASE test_2;
USE test_2;
SELECT User FROM mysql.user;
[root@monitor sql]# cat query1.sql query2.sql | mysql -uroot -ppassword
User
root
root
User
root
root
[root@monitor sql]# 
```

For the path to mysql how would you like to see it dealt with ?