Correct name of image built-in properties

According to glance image v1 and v2 speces [0][1], two image property
names should be corrected:

* "minDisk" => "min_disk"
* "minRam" => "min_ram"

[0] http://developer.openstack.org/api-ref-image-v1.html
[1] http://developer.openstack.org/api-ref-image-v2.html

Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>