Allow certificate_key to be specified like account_key

[dnet]

What

I added an option to the Client constructor named certificate_key which behaves just like the optional parameter account_key -- it allows the user to specify a private key instead of generating one.

Why

While I'm all for generating new keys upon each renewal as it dramatically lowers the time window in which a leaked key can be abused, there are situations when a pre-existing key must be used.

For example if HPKP is already set up and clients expects private keys to be selected from a limited pool already advertised in the header, there's no other option but to generate a certificate that matches at least one of the entries already known by User-Agents.

[komuw]

@dnet
You would also need to add it to the command line app:

sewer/sewer/cli.py

Line 132 in 4547715

if account_key:

[codecov]

Codecov Report

Merging #93 into master will decrease coverage by 0.18%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master     #93      +/-   ##
=========================================
- Coverage   93.39%   93.2%   -0.19%     
=========================================
  Files           6       6              
  Lines         469     471       +2     
=========================================
+ Hits          438     439       +1     
- Misses         31      32       +1

Impacted Files	Coverage Δ
sewer/client.py	93.6% <66.66%> (-0.3%)	⬇️

[dnet]

You would also need to add it to the command line app

OK, I added that in c34a91b. CircleCI seems to fail on that, but I cannot comprehend its error message. It seems like some kind of internal error during the flake8 test, however flake8 reports no problems on my machine. (And the CircleCI output only contains stack traces showing that it couldn't run flake8...)

[komuw]

@dnet Thanks for working on this.

ci is failing because of this upstream bugs in flake8:

• PyCQA/pycodestyle#741
• https://gitlab.com/pycqa/flake8/issues/406
• https://gitlab.com/pycqa/flake8/issues/415