Addressing review from @ryan-lane. exists will now raise a SaltInvocationError when arguments are not specified.
Added missing check for arguments in subnet_exists.
Added missing check for arguments in dhcp_options_exists.
Added missing check for arguments in network_acl_exists and fixed DHCP options test.
Added missing check for arguments in route_table_exists().
Addressing review from @ryan-lane. Reduced log level in __virtual__ to debug.