Added acquire_ticket_vm API to retrieve a screen ticket

Added acquire_ticket_vm API to retrieve a screen ticket for VMRC browser plugin

* API Documentation:

 http://pubs.vmware.com/vcd-55/index.jsp#com.vmware.vcloud.api.doc_55/GUID-0DF63045-16D7-49E7-878E-B56E27F99C62.html

 http://pubs.vmware.com/vcd-55/index.jsp?topic=%2Fcom.vmware.vcloud.api.reference.doc_55%2Fdoc%2Foperations%2FPOST-AcquireTicket.html

* Implementation example:

 https://www.vmware.com/support/vcd/doc/vcd-1.5-vmrc-api-example.zip

* VMRC documentation:

 https://www.vmware.com/pdf/vcd_15_vmrc_api.pdf