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 is still writable from within my test container:

```
# test -w /proc
# echo $?
0
```

If I drop all the way down to `swappiness`, then the test fails:

```
# test -w /proc/sys/vm/swappiness
# echo $?
1
```

You may want to base the check on whatever the `sysctl` error message can't write to.