Added means to add arbitrary ACL rules to the webinterface

Hi,

I wanted to **not** have ACL rules for groups (eg, "allow @sudo") for the webinterface and, as it was, the role did not allow me to do that since the defaults include these values. As such, I've removed it from the defaults and change the tasks that create the groups such that they'll only be created when the vars `monit_webinterface_r_group`and `monit_webinterface_rw_group` are defined.

Also, I've had a `monit_webinterface_acl_rules` var, so that we can add ACL rules in the form of "allow IP-ADDRESS" or others.

Thanks