Init sphinx config values before accessing them.

Sphinx's config object needs to have its values initialized with
init_values for the values to be loaded from _raw_config into config.
Without this pbr gets null equivalent values from Sphinx's config. These
null values prevented man pages from being built when Sphinx warnerrors
is set. Fix this.

Change-Id: I0f17ca6eff036a0bcc6686fcc3f4efe0830cc585