Double quote the csv plugin DataDir.

I tried using the csv plugin _without_ this on CentOS6 with collectd-5.4.1, but got:
```
$ sudo service collectd restart
Parse error in file `/etc/collectd.d/10-csv.conf', line 27 near `/': syntax error, unexpected SLASH
yyparse returned error #1
configfile: Cannot read file `/etc/collectd.d/10-csv.conf'.
Unable to read config file /etc/collectd.conf.
Error: Reading the config file failed!
Read the syslog for details.
not restarting due to configuration error
                                       [FAILED]
```