NIST: increase RSA key length to 2048 bit

According to NIST 800-131A, RSA key lenght for digital signature
must >= 2048 bit. Now we use 1024 bit key to generate x509 cert
file. Need to increase the key length to 2048 bit.

Change-Id: I59f614b5d8a79f9e0a96503867cfca176be5c757
Closes-Bug: 1369487

nova/crypto.py

@@ -322,7 +322,7 @@ def _user_cert_subject(user_id, project_id):
     return CONF.user_cert_subject % (project_id, user_id, timeutils.isotime())
 
 
-def generate_x509_cert(user_id, project_id, bits=1024):
+def generate_x509_cert(user_id, project_id, bits=2048):
     """Generate and sign a cert for user in project."""
     subject = _user_cert_subject(user_id, project_id)