Use signing digest defined in config to sign CSR

Replace the hardcoded signing digest with the one as retrieved from
the corresponding configuration attribute.

Change-Id: Id51f44074ce0495609d6c8d99bae8cad7f32057f

octavia/certificates/generator/local.py

@@ -180,7 +180,8 @@ class LocalCertGenerator(cert_gen.CertGenerator):
             ])).add_extension(
             x509.BasicConstraints(
                 ca=False, path_length=None), critical=True,
-        ).sign(pk, hashes.SHA256(), backends.default_backend())
+        ).sign(pk, getattr(hashes, CONF.certificates.signing_digest.upper())(),
+               backends.default_backend())
         return csr.public_bytes(serialization.Encoding.PEM)
 
     @classmethod