Allow option to not change autocommit setting

This way we can install redis on a container (where it might not be possible to change kernel settings).

Otherwise, if you use it on a container (e.g. a proxmox/openvz container) you will get an erroe of the form:
```
TASK: [bennojoy.redis | Set the kernel paramter for vm overcommit] ************
failed: [hostname.test] => {"failed": true, "item": ""}
msg: checks_before failed with: key_path is not a writable file, key seems to be read only

FATAL: all hosts have already failed -- aborting
```
This way, the default stays the same as before, but if we want we can not have the role try to change a kernel setting.