Add certificate manager mechanism for VPNaaS IPSec connections

This spec introduces a certificate manager mechanism to vpn as a Service (VpnaaS).
In other words，it bring in a new function to support certificate for authentication
when creating VPN IPSec site-to-site connections. As we know, creating IPSec site-to-site
connections only support PSK for authentication and main mode in ike_phase1_mode,
this ipsec connection cannot fulfill authentication successfully in NAT schema.
So we add a certficate manage mechanism to manager cerficate and use certificate
for authentication to solve the problem in NAT schema.