Make keystone_pki less keystone specific

We need to create ssl certs for services other than Keystone. Renaming
keystone_pki to ssl_pki, and making the routines less keystone specific
so we can create certs for other services.

Also creating generate_ssl_cert command which can generate ssl
certificates for multiple services using a single CA.

Change-Id: I4d65cceee9e15ce0ca8ef309127a587135355810
Merge "Make keystone_pki less keystone specific"