Create diff generation method

[kbreit]

Much of the diff creation code in the modules is duplicated. The Meraki class should have a generate_diff method which does the work in a centralized and consistent manner

• meraki_admin.py
• meraki_config_template.py
• meraki_content_filtering.py
• meraki_device.py
• meraki_firewalled_services.py
• meraki_malware.py
• meraki_mr_l3_firewall.py
• meraki_mx_l3_firewall.py
• meraki_mx_l7_firewall.py
• meraki_nat.py
• meraki_network.py
• meraki_organization.py
• meraki_snmp.py
• meraki_ssid.py
• meraki_static_route.py
• meraki_switchport.py
• meraki_syslog.py
• meraki_vlan.py
• meraki_webhook.py

[kbreit]

All modules supporting diff have been converted and will be pushed, with one exception. meraki_nat uses a more complex diff data structure which won't be appropriate for the method.