Enable nova server to be run in SSL mode

This commit allows one to specify ca, cert and key file
to run nova server in SSL mode.

Note: The flag use_ssl per se is not used in nova yet,
its purpose here it to verify collateral parameters.

Change-Id: I5aed08afc2b6ac94bf9e1929f6b1f41a88882f02
(cherry picked from commit 31048d2de9160ceb1da76c96ae8a38257e9ea577)