[package] add a requirements file with execo>=2.2
[bin] set matplotlib backend
[packages] boot_time now use dir as argument and read correcly the vms.list file
[lib] actions: change test on line to destroy vms
[lib] deployment check on host is optionnaly strict, fix a wrong init in _get_xml_elements
[packages] boot_time list correctly the vms
[lib] improve a bit topology plots