Feature: Allow specifying user (and group) for sync group and instance notify scripts

[Salvoxia]

Use Case

Run keepalived with enable_script_security and default user keepalived_script to run scripts with, but still run some notify scripts with elevated permissions.

Implementation

Added support for the following optional keys in keepalived_sync_groups and keepalived_instances to control the user (and group) a notify script runs with:

keepalived_sync_groups:
  group:
    notify_script_user_group: user [group]
    notify_master_user_group:  user [group]
    notify_backup_user_group: user [group]
    notify_fault_user_group: user [group]
keepalived_instances:
  instance:
    notify_script_user_group: user [group]
    notify_master_user_group: user [group]
    notify_master_rx_lower_pri_user_group: user [group]
    notify_backup_user_group: user [group]
    notify_fault_user_group: user [group]
    notify_stop_user_group: user [group]

Implements #295