Add 'cryptography' package to test-requirements.txt

When running the functional tests locally:

  tox -efunctional

the following error is raised from 6 tests in the
placement.tests.functional.db.test_migrations module:

  RuntimeError: 'cryptography' package is required for sha256_password
  or caching_sha2_password auth methods

This adds the 'cryptography' package to the list of required packages
for tests.

Story: 2008832
Task: 42317

Change-Id: I78fbdd723be089364b8ee61432eeb74ed6f24094

lower-constraints.txt

@@ -4,6 +4,7 @@ attrs==17.4.0
 bandit==1.1.0
 colorama==0.3.9
 coverage==4.0
+cryptography==2.7
 debtcollector==1.19.0
 decorator==3.4.0
 extras==1.0.0

test-requirements.txt

@@ -19,6 +19,7 @@ bandit>=1.1.0 # Apache-2.0
 gabbi>=1.35.0 # Apache-2.0
 
 # placement functional tests
+cryptography>=2.7
 wsgi-intercept>=1.7.0 # MIT License
 
 # needed to generate osprofiler config options