Custom CA certificate for verifying SSL connections

The various OpenStack client libaries support specifying a SSL
certificate that should be used to verify the SSL-connection, but
this option was not exposed in the horizon options. This patch
implements that. Note that although some clients do implement a
cacert-parameter, they do not necessarily actually verify the
certificate (Swift).

Change-Id: I9dfbd48a0d96103421da7d884e839d591a90cab1
Implements: blueprint ssl-cacert
Merge "Custom CA certificate for verifying SSL connections"