[LIBCLOUD-1001] Update Cloudflare DNS driver to API v4

[c-w]

Update Cloudflare DNS driver to API v4

Description

As discussed in LIBCLOUD-1001, Cloudflare turned off the old JSON API and replaced it with the new v4 API. This pull request updates the Cloudflare DNS driver to use the new API.

Note that in order to keep the implementation of the v4 driver streamlined, none of the existing ex_ methods were migrated to the new API.

Note that LIBCLOUD-994 is also fixed by the new driver implementation.

Status

• done, ready for review
• CI passed

Checklist (tick everything that applies)

• Code linting
• Documentation
• Tests
• ICLA

[codecov-io]

Codecov Report

Merging #1292 into trunk will increase coverage by <.01%.
The diff coverage is 96.32%.

@@            Coverage Diff            @@
##           trunk    #1292      +/-   ##
=========================================
+ Coverage   85.9%   85.91%   +<.01%     
=========================================
  Files        365      365              
  Lines      75137    75056      -81     
  Branches    6858     6861       +3     
=========================================
- Hits       64547    64484      -63     
+ Misses      7825     7809      -16     
+ Partials    2765     2763       -2

Impacted Files	Coverage Δ
libcloud/test/dns/test_cloudflare.py	100% <100%> (+1.42%)	⬆️
libcloud/dns/drivers/cloudflare.py	95.18% <93.87%> (+5.75%)	⬆️

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[c-w]

Tagging @Kami as the original author of the Cloudflare DNS driver.

[Kami]

@c-w Thanks for the contribution.

Since it's a bigger change, signed ICLA is required (http://libcloud.readthedocs.org/en/latest/development.html#contributing-bigger-changes).

[c-w]

Thanks for the patience @Kami. The ICLA is now signed.

[Kami]

LGTM.

[Kami]

I think it would be good to release v2.6.0 in the near future once this PR and a couple of other ones are merged.

Do you want to try the release process for v2.6.0?

[c-w]

Thanks for the review @Kami and yes, I'd be happy to go through the release flow for v2.6.0.