server/container_create: Factor out setupCapabilities helper
server/container_create: Factor out setupCapabilities helper

Having a separate function holding the details of this makes reading
createSandboxContainer easier.

While I was moving the code, I've also cleaned up two things:

* The nil capabilities check is now earlier, where before it had been
  between the ALL handling and the non-ALL handling.

* I've added a capPrefixed variable to avoid having multiple
  toCAPPrefixed calls per capability.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

vbatts/cri-o,kubernetes-incubator/cri-o,vbatts/cri-o,nalind/cri-o,rhatdan/cri-o,mikebrow/cri-o,14rcole/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/ocid,kubernetes-incubator/ocid,14rcole/cri-o,kubernetes-incubator/cri-o,vbatts/cri-o,mikebrow/cri-o,mikebrow/cri-o,vbatts/cri-o,14rcole/cri-o,jwhonce/ocid,mikebrow/cri-o,nalind/cri-o,jwhonce/ocid,jwhonce/ocid,kubernetes-incubator/cri-o,rhatdan/cri-o,14rcole/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,rhatdan/cri-o,rhatdan/cri-o