Fix storage type fixtures

[atrautsch]

I changed the fixtures and storage volume type to singular.
That should be correct according to the current swagger API example.

However, I do not know if this was always the case.

[stgraber]

Can you edit your commit message to include the Signed-off-by: Firstname Lastname <email> line?

You can usually do so with git commit --amend -s followed by a git push --force

[codecov-commenter]

Codecov Report

Merging #497 (2a917c1) into master (1bbd68e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #497   +/-   ##
=======================================
  Coverage   97.07%   97.07%           
=======================================
  Files          56       56           
  Lines        4101     4101           
=======================================
  Hits         3981     3981           
  Misses        120      120           

Impacted Files	Coverage Δ
pylxd/tests/mock_lxd.py	100.00% <ø> (ø)
pylxd/models/storage_pool.py	100.00% <100.00%> (ø)

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[stgraber]

Thanks!