Don't try to regenerate existing ssl certificates

Rerunning stack.sh after some failure unrelated to ssl setup will fail
due to certificates already existing in the CA index. Don't regenerate
them instead. This is a workaround making devstack development easier
rather than something typical user would run into.

Change-Id: Icfd4cb5132c8c9297eb73159e592b7006295184f
Merge "Don't try to regenerate existing ssl certificates"