Use SHA256 instead of MD5 in completion cache

FIPS 140 are U.S. government computer security standards that specify
requirements for cryptography modules. MD5 is not FIPS compliant [1].
Previously, MD5 was used as the hash algorithm for the bash completion
cache. Hosts running in FIPS mode [2] block execution of the MD5 hash.
This makes python-novaclient unusable on FIPS-enabled machines. This
patch replaces MD5 with SHA256, which is FIPS compliant.

[1] https://csrc.nist.gov/projects/hash-functions
[2] https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/security_guide/chap-federal_standards_and_regulations

Change-Id: Ia8750bc27aa9a2cfafb6f4f49252f5bd81bc1a40
(cherry picked from commit 2595bac)

novaclient/base.py

@@ -307,8 +307,8 @@ def completion_cache(self, cache_type, obj_class, mode):
             # endpoint pair
             username = utils.env('OS_USERNAME', 'NOVA_USERNAME')
             url = utils.env('OS_URL', 'NOVA_URL')
-            uniqifier = hashlib.md5(username.encode('utf-8') +
-                                    url.encode('utf-8')).hexdigest()
+            uniqifier = hashlib.sha256(username.encode('utf-8') +
+                                       url.encode('utf-8')).hexdigest()
 
             cache_dir = os.path.expanduser(os.path.join(base_dir, uniqifier))