Fix curl options for downloading kubectl

[vorakl]

At the moment, the command for downloading kubectl binary has a hidden but real issue.
"--progress-bar -LO" options make it look good but curl won't fail if, by any chances, it is not able to download a binary. It's even worse, curl does create a file which contains an error message and is named 'kubectl'.

$ curl --progress-bar -LO https://storage.googleapis.com/kubernetes-release/release/v000000/bin/darwin/amd64/kubectl
####################################################################################################################################################################### 100.0%

$ cat kubectl 
<?xml version='1.0' encoding='UTF-8'?><Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Details>No such object: kubernetes-release/release/v000000/bin/darwin/amd64/kubectl</Details></Error>

On the next step, the script moves this "binary" to /usr/loca/bin/, assigns an executable flag and an end user eventually will run this xml file instead of real kubectl tool.
What it really has to do is to fail (return non 0 exit code) and do not create any file at all.

Of course, there is another, global problem, but it's not bound to the scope of this issue.
A bigger problem is that, it doesn't matter curl creates a file with a wrong content or it doesn't, the script it self won't react on a non-zero exit code of any commands and nothing stops it from going further. So, if the script is not able to download a working copy of binary, it won't tell anyone about that. It will be a surprise for user later ;)

Anyway, I think, it's another issue and it's a subject to discussion in another thread.