Optional GSSAPI auth for Custodia client

[tiran]

PR based on #235

[codecov-io]

Codecov Report

Merging #238 into master will decrease coverage by 0.08%. The diff coverage is 61.76%.

@@            Coverage Diff            @@
##           master    #238      +/-   ##
=========================================
- Coverage   72.79%   72.7%   -0.09%     
=========================================
  Files          41      41              
  Lines        4440    4470      +30     
  Branches      449     454       +5     
=========================================
+ Hits         3232    3250      +18     
- Misses       1052    1063      +11     
- Partials      156     157       +1

Impacted Files	Coverage Δ
tests/test_custodia.py	95.2% <100%> (+0.17%)	:arrow_up:
src/custodia/cli/__init__.py	44.32% <40%> (-1.08%)	:arrow_down:
src/custodia/client.py	79.14% <50%> (-1.15%)	:arrow_down:

---

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 511bf2a...1086d5b. Read the comment docs.

[tiran]

Travis CI is currently broken:

$ sudo apt-get -qq update
W: GPG error: http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.4 Release: The following signatures were invalid: KEYEXPIRED 1515625755
W: The repository 'http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.4 Release' is not signed.
W: http://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/Release.gpg: Signature by key 15866BAFD9BCC4F3C1E0DFC7D69548E1C17EAB57 uses weak digest algorithm (SHA1)
E: Failed to fetch store:/var/lib/apt/lists/partial/repo.mongodb.org_apt_ubuntu_dists_trusty_mongodb-org_3.4_multiverse_binary-i386_Packages  Empty files can't be valid archives
E: Some index files failed to download. They have been ignored, or old ones used instead.
``

[tiran]

@adelton please test and confirm this PR

[tiran]

thx

I'll merge the PR as soon as @adelton has confirmed that it's working for him.

[adelton]

I've backported the patch to 0.5 to test on my 0.5 setup, I tried it both with --gssapi and without it on machine where I've forcibly removed python-requests-gssapi, things look good. Thank you for making it optional.

[tiran]

thx for testing!