Gitian builds on all platforms using Vagrant/VirtualBox (GH #1597)

Wladimir points out that Virtualbox has lower CPU requirements and can be used on older CPUs, unlike KVM which is slower without hardware virtualization. Furthermore, KVM and Virtualbox do not function well together.Mark shares his own experience with setting up and maintaining his gitian build system, expressing difficulties in doing so. To address this problem, he created a makefile that simplifies the process. Now, performing a gitian build only requires running "cd contrib/vagrant && make" on any platform, as long as the necessary dependencies are installed. These scripts have been submitted by Mark as pull-request #1597 on Github, with the hope that others will find them useful.The email thread concludes with a mention of an exclusive live event that will delve into today's security and threat landscape, covering topics such as endpoint security, mobile security, and the latest malware threats. The email also includes links to the Bitcoin-development mailing list for further reference.