rpc: allow overriding NSS DB directory in API config

Add new `nss_dir` API config option to allow rpcclient to use a non-default
NSS DB for the connection.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/constants.py

@@ -129,6 +129,8 @@ DEFAULT_CONFIG = (
 
     ('rpc_protocol', 'jsonrpc'),
 
+    ('nss_dir', paths.IPA_NSSDB_DIR),
+
     # Define an inclusive range of SSL/TLS version support
     ('tls_version_min', 'tls1.0'),
     ('tls_version_max', 'tls1.2'),