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/tests/test_validators.py

@@ -0,0 +1,29 @@
+from marshmallow.exceptions import ValidationError
+from .vectors import PRIVATE_KEY_STR
+
+
+def test_private_key():
+    from lemur.common.validators import private_key
+    try:
+        private_key(PRIVATE_KEY_STR)
+        assert True
+    except ValidationError:
+        assert False, "failed to validate private key as a bytes object"
+
+
+def test_private_key_str_object():
+    from lemur.common.validators import private_key
+    try:
+        private_key(PRIVATE_KEY_STR.decode('utf-8'))
+        assert True
+    except ValidationError:
+        assert False, "failed to validate private key as a str object"
+
+
+def test_private_key_invalid():
+    from lemur.common.validators import private_key
+    try:
+        private_key('invalid_private_key')
+        assert False, "invalid private key should have raised an exception"
+    except ValidationError:
+        assert True