Enable heat server to be run in SSL mode

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

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

Change-Id: I94d0b461adc752b028770aea71bf4e4612722539
(cherry picked from commit 025b8cb830d7fa476c4ab1a0b0228b88b5ba10c3)