Verify SSL certificates at boot time

When Glance is configured to run with SSL this patch will check
that the certificate and key exist, are readable, and match each
other at boot time.  If any of the above conditions fail an friendly
error message is logged directing the operator at a solution.
Previously the keys were not checked until a client connected.

fixes bug: 1160529

Change-Id: I16975608f9ae40ac5b84b3caa52ed5f8e22f296e
Merge "Verify SSL certificates at boot time"