Avoid ipv6 when installing agent on CirrOS

Not sure why, but CirrOS's wget (i.e., busybox) likes to use ipv6 addresses when
resolving host names. Since busybox's wget doesn't have a -4 flag, which forces
ipv4, we use curl instead, which seems to prefer ipv4 by default.

Change-Id: I86a5187d15d81868856fbf49303f064a646e331f