ConfigFile security: Migrate Proxy password to keychain

ConfigFile security: Migrate Proxy password to keychain

When specified in the config file, the Proxy password will be migrated
to the keychain, for backward compatibility and to allow admins to
overwrite an existing password by rolling out updated config files.

Once migrated to the keychain, the password will be removed from the
config file.

Signed-off-by: Michael Schuster <michael@schuster.ms>