CIDR fixes and kops support

[gianlucaborello]

This PR introduces two changes, split in separate commits:

1. Better validation of IP addresses to CIDR, since the original algorithm was a little bit weak, and it also adds some tests.

2. Includes the 100.64.0.0/10 address range as valid range.

The reason for 2) is that kops by default seems to be using that range to minimize the likelihood of collisions in the wild[1] . This is in my opinion improper and against the guidelines of the RFC (e.g. [2], [3]), but considering the popularity of kops it's a scenario we have to support, so we either introduce a new configuration parameter inside the CRD that the user can use to manually specify the whitelist range (and this would then be passed to the mysql-agent as argument when the statefulset is first created), or we just hardcode the address range, like I did. The latter has the advantage that it will work out of the box, which is a usability advantage with respect to the other solution in my opinion.

Thanks

[1] kubernetes/kops#2075
[2] https://blog.ipspace.net/2013/08/can-i-use-shared-rfc-6598-ipv4-address.html
[3] https://networkengineering.stackexchange.com/questions/35958/ipv4-segment-100-64-0-0-10

[prydie]

LGTM. Thanks; great work as ever!