Curl is more ubiquitous than wget. For instance, the GCE centos-7 and
rhel-7 image families ship curl by default, but not wget.

Looking at the shell scripts under cluster/, they tend to use curl more
than wget. (The ones that use wget, such as get-kube.sh, try curl first
and only fallback to wget if it's not available.)

Tested: by running node-e2e-test on Ubuntu, COS and CentOS.