Remove hardcoded login url in session creation code
vCloud provides and API to retrieve the login URL. We should use that instead of hardcoding "/api/sessions" in vcd_client.rb

Refer to:
http://pubs.vmware.com/vcd-55/index.jsp?topic=%2Fcom.vmware.vcloud.api.doc_55%2FGUID-ED747CDC-C9B8-48C5-98D9-B869EFF75ABD.html

http://pubs.vmware.com/vcd-55/index.jsp#com.vmware.vcloud.api.doc_55/GUID-6DC15CF5-3BCF-4426-9988-C71E7A71CBD6.html#GUID-6DC15CF5-3BCF-4426-9988-C71E7A71CBD6

Change-Id: I2a799ae981ba307cbde7f642a24262975e041750