feature #3180: options for SSL certs in OCA

ONE_CERT_DIR: adds an extra directory with trusted CA certificates
ONE_DISABLE_SSL_VERIFY: disable certificate verification

Both of these options make the calls change from asynchronous (one http
connection per call) to synchronous (same http connection for all calls).
XMLRPC library creates a new HTTP object per asynchronous connection and
there is no way of passing configuration options to it.
feature #3180: add option to make xmlrpc calls synchronous

Using SSL options in Ruby OCA no longer disable async calls automatically. The sync call
must be specified when creating a client:

client = OpenNebula::Client.new(secret, proxy, :sync => true)
feature #3180: make CLI use sync calls to honor SSL parameters
feature #3180: Raise an exception for not compatible client options
Merge branch 'feature-3180'