UPGRADE: do not throw error if no sites are configured
Previously, if 'rb-site upgrade --all-sites' was called and the

site file either did not exist or contained no valid entries (such

as all sites having been removed but the sitelist being missed),

it would return with error code 1.
This breaks scripts such as that on an upcoming Fedora release of

Review Board that will be running the --all-sites upgrade as part

of the Apache start-up. In order for it to be possible for Review

Board to be installed on the system, unconfigured and not break

Apache, we need to treat this exit code as success.

Verified that running 'rb-site upgrade --all-sites' with no /etc/reviewboard/sites file printed the message "No Review Board sites listed in /etc/reviewboard/sites" and exited with code 0 (success).