add neutron-db-check command

The neutron-db-check command will check the version of the
Neutron database against a configured value in
/etc/sysconfig/openstack-neutron, and if they do not match will log an
error to syslog (and stderr) and exit with an error code.
call neutron-db-check from neutron-server init script

this modifies the neutron-server init script to call neutron-db-check
and refuse to start neutron-server if the database is not at the right
version.
install neutron-db-check

modifies openstack-neutron.spec to install the neutron-db-check command
and associated sysconfig file.