Fixed adding SSL key

Fixes bug that made impossible to add new ssl key.

```
root@maggie:~# cat server.key | dokku ssl:key maggie
Error opening Private Key privateKey.key
139883262244512:error:0200100D:system library:fopen:Permission denied:bss_file.c:398:fopen('privateKey.key','r')
139883262244512:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:400:
unable to load Private Key
```

is now

```
root@maggie:~# cat server.key | dokku ssl:key maggie
RSA key ok
writing RSA key
-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----
-----> Deploying maggie ...
-----> Shutting down old containers
=====> Application deployed:
```