Fix import certificate private key encoding (#434)

When importing a certificate, the private key is passed to the
import/upload process from the UI as a str object. In Python3 this
raises two issues when processing the private key - the private key
validation fails and database insert of the certificate fails.

The fix in both cases is to correctly encode the private key as a bytes
object.

lemur/certificates/service.py

@@ -189,6 +189,11 @@ def upload(**kwargs):
     else:
         kwargs['roles'] = roles
 
+    if kwargs.get('private_key'):
+        private_key = kwargs['private_key']
+        if not isinstance(private_key, bytes):
+            kwargs['private_key'] = private_key.encode('utf-8')
+
     cert = Certificate(**kwargs)
 
     cert = database.create(cert)