add a api for the adminitrator to delete host infos

In practical applications, the hosts should be removed or damaged.

But there has no api to remove host infos. After the host is removed or damaged, the 'nova service-list' will always get the host info which status is down. So there should be an api for the system administrator to delete the removed or damaged host, and the administrator can use the api to stop nova-compute process before removing the host, too.

such as:
nova service-list
+------------------+-----------+----------+---------+-------+----------------------------+
| Binary | Host | Zone | Status | State | Updated_at |
+------------------+-----------+----------+---------+-------+----------------------------+
| nova-cert | openstack | internal | enabled | up | 2013-12-16T10:46:29.000000 |
| nova-compute | compute02 | az1 | enabled | up | 2013-12-16T10:46:27.000000 |
| nova-compute | compute01 | az2 | enabled | down | 2013-12-02T22:29:00.000000 |
+------------------+-----------+----------+---------+-------+----------------------------+

The compute01 has been removed, but there is no way to remove the record.