Exposed the cert value to all TAXII endpoints

[zrush-mitre]

No description provided.

[zrush-mitre]

Closes #86 ?

[codecov-io]

Codecov Report

Merging #89 (b0abfdc) into master (1135ca4) will decrease coverage by 0.04%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   95.27%   95.23%   -0.05%     
==========================================
  Files           8        8              
  Lines        1863     1867       +4     
==========================================
+ Hits         1775     1778       +3     
- Misses         88       89       +1     

Impacted Files	Coverage Δ
taxii2client/common.py	81.87% <66.66%> (-0.44%)	⬇️
taxii2client/v20/__init__.py	91.10% <100.00%> (+0.02%)	⬆️
taxii2client/v21/__init__.py	91.56% <100.00%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1135ca4...b0abfdc. Read the comment docs.

[zrush-mitre]

This has been confirmed to work as expected using the PFTE as testing. I can't really think of a way to add a test to the testbed, since I don't know of a way to emulate SSL in a test file (I could do the encryption/decryption of stuff manually in a test, but I don't have a great understanding of exactly how it works on a practical level).