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
Merge "Allow deploying keystone with SSL certificates"
Merge branch 'master' of github.com:openstack-dev/devstack