It seems that wget throws some errors from time to time when getting
content from tarballs.openstack.org.
The errors are SSL-related, maybe some interaction between the SSL libaries
of the libguestfs appliance and the load balancing configuration
of tarballs.openstack.org (SNI-based).
curl seems to be immune, so let's use it.
Story: 2003961
Task: 26890
Change-Id: Ia5793a4be8cde352329e861c46de8c4eeac4312e
12b270b092