Remove un-used JSON attribute 'private_key_id'. Clarify format of 'private_key' parameter.

Would be nice if characters before `-----BEGIN PRIVATE KEY-----` would be ignored.

My file starts like
```
Bag Attributes
    friendlyName: privatekey
    localKeyID: A4 66 6F 63 25 31 34 31 31 30 34 39 33 38 32 36 32 34 
Key Attributes: <No Attributes>
-----BEGIN PRIVATE KEY-----
```
and I get an exception that the file is expected to start with `-----BEGIN PRIVATE KEY-----`