Multiple network resource and data source improvements

[DevOpsFu]

This PR fixes a few things with the Network resource and data source:

• Fix the ipv6_ra_enabled option name (it was previously ipv6_ra_enable which does not match the JSON field name returned by the API.
• Allow control of the DHCP Guarding option.
• Allow the DHCP server IP addresses to be specified (this can be used in conjunction with DHCP guarding)
• Remove the call to cidrZeroBased on the IP Subnet. The subnet field is also used to specify the IP address which is allocated to the USG itself, and this was always being normalized to zero.
• Control which settings are updated based on the IPV6 interface setting. If it is set to none then the IPV6 settings are ignored. This removes the need to specify unused IPV6 settings to prevent unstable plans.

[DevOpsFu]

@paultyng @joshuaspence Is this repo still being maintained?