Drop implementation to use pyOpenSSL to manage pkcs12 certs

The cryptography library has been bumped to 3.1 in upper-constraints
file during Ussuri, which is quite old. So we no longer have to
maintain logic for cryptography < 3.0.

Change-Id: I1a463e320b94b0e99e92541581e1ee5feffd356a
diff --git a/requirements.txt b/requirements.txt
index 7b2f0a2..afa0565 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,7 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 
-cryptography>=2.1 # BSD/Apache-2.0
+cryptography>=3.0 # BSD/Apache-2.0
 python-dateutil>=2.5.3 # BSD
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 oslo.config>=5.2.0 # Apache-2.0