import_precheck cli tool fail to check Ubuntu 18.04 version

[paoloteti]

Even if Ubuntu 18.04 LTS is listed in the official documentation (section importing virtual disks) as a supported OS, for the import_precheck cli tool this version it is unknown since not listed in check_osversion.go

var osVersions = map[string][]string{
	"centos":  {"6", "7"},
	"debian":  {"8", "9"},
	"ol":      {"6", "7"},
	"rhel":    {"6", "7"},
	"ubuntu":  {"14.04", "16.04"},
	"windows": {"6.1", "6.3", "10.0"},
}

Running import_precheck on a Ubuntu 18.04 LTS image all checks complete with success, except this one.