Fix centos kickstart_done snippet

Fix centos kickstart_done snippet

Bug:
CentOS-7-Minimal-1611-x86_64 image doesn't contain
wget package and kickstart_done snippet use wget to
call cobbler api to set netboot enable to false.
This way if host installed centos which is not set
to boot from disk will stuck in pxe loop

Solution:
Use curl instead of wget in kickstart_done

Change-Id: I0414adab72047a367b7cb15fe6a5537c76234972
Signed-off-by: Harry Huang <huangxiangyu5@huawei.com>