curl to wget in verify image service installation

Changing "curl" to "wget" in "Verifying the Image Service installation"
 page. This is because Ubuntu doesn't include curl in the default
base/minimal installation.

Change-Id: I16364d05085a2a9c3713e94f71a3250215361540
Closes-Bug: #1248605
backport: havana
(cherry picked from commit de0eb7db9081743f7e46d853056f66b251c047b0)
Merge "curl to wget in verify image service installation" into stable/havana