Ignore errors if sysctl fails; needed for running in restricted env

When provisioning Riak via Ansible in a constrained container environment, like modern version of Docker, access to `/proc` and `/sys` is locked down, which prevents `sysctl` from operating.

As far as I can tell, the `sysctl` commands here (and in `ansible-riak-common`) don't affect the installation, but are performance improvements.

So, ignoring errors here seems safe.

Let me know if this is reasonable (or if there is a better way). If so, I can make a similar pull request on `ansible-riak-common`.
This strange. In my Docker image, `/proc` is read-only. I'll change this to just be `/proc/sys/vm/swappiness` then.