Add haproxy_disable_restart variable

[stuwil]

Adds the haproxy_disable_restart (default: false) variable to conditionally skip the restart haproxy handler, for cases where this might not be wanted (e.g. a reload instead).

[r-daneel]

I would suggest another approach:
Rename the notification from 'restart haproxy' to 'reload configuration'.
The handler would then use a variable named 'config_change_action' that could be 'reload', 'restart' or 'false' that would default to 'restart' (for backwards compatibility).
In this way, the user can choose the action instead of just having an all-or-nothing switch