Allow deploying keystone with SSL certificates

Allow providing certificates through environment variables to be used
for keystone, and provide the basis for doing this for other services.
It cannot be used in conjunction with tls-proxy as the service provides
it's own encrypted endpoint.

Impletmenting: blueprint devstack-https
Change-Id: I8cf4c9c8c8a6911ae56ebcd14600a9d24cca99a0