Update version string to 2.0.7-pre
Create .gitingore file
Create README file under root dir of resource disk

The README file is used to warn user that the resource disk is a
temporary storage.
Fix DoDhcpWork failed issue

In each retry loop, try to open 68 port for DHCP UDP traffic.
Fix string concat issue in error log.
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.