Fix host key not found bug

In ubuntu, cloud init does the provisioning work. In waagent, it should
wait until the ssh host key is generated before report it to azure.
Handle 'device already mounted' situation

In ubuntu img, the resource disk is mounted by fstab. In this case, get
the mount point and create README under the root.
Fix extension status report issue during provision

If no extension is installed, report empty status list. So that the
portal will stop query extension status and show the correct provision
state.