Add CertManager to store CA and client certificate

To implement TLS support, we should store CA and client cert for each
bay. This patch adds common library to store cert to Barbican.
Magnum uses service admin privilege to store the cert, this means that
end user can't retrieve CA cert and private key from Barbican
directly.

This patch is copied from neutron-lbaas project.
*  I435189b2637e32803a13ebd4951e61fac4ab234d

Change-Id: I519228d9749ad610db3e0c698caa1144813f9d52
Partial-Implements: blueprint magnum-as-a-ca

etc/magnum/magnum.conf.sample

@@ -330,6 +330,20 @@
 #bay_create_timeout = <None>
 
 
+[certificates]
+
+#
+# From magnum
+#
+
+# Certificate Manager plugin. Defaults to barbican. (string value)
+#cert_manager_type = barbican
+
+# Absolute path of the certificate storage directory. Defaults to
+# /var/lib/magnum/certificates/. (string value)
+#storage_path = /var/lib/magnum/certificates/
+
+
 [conductor]
 
 #