Cache external tool versions

[simondeziel]

During its operation, the LXD daemon often end up checking the versions of external tools like apparmor_parser, dnsmasq, nft, xtables, QEMU, rsync, rbd and radosgw-admin. Doing those ${BIN} --version calls is not free so the obtained version information should be cached after the first version check or maybe at LXD daemon start.

The list of such version checks can be obtained with this git grep:

$ git grep 'Command.*"--version"'
lxd/apparmor/apparmor.go:       out, err := shared.RunCommand("apparmor_parser", "--version")
lxd/dnsmasq/dnsmasq.go: output, err := shared.RunCommandCLocale("dnsmasq", "--version")
lxd/firewall/drivers/drivers_nftables.go:       output, err := shared.RunCommandCLocale("nft", "--version")
lxd/firewall/drivers/drivers_xtables.go:        output, err := shared.RunCommandCLocale(cmd, "--version")
lxd/instance/drivers/driver_qemu.go:    out, err := exec.Command(qemuPath, "--version").Output()
lxd/rsync/rsync.go:     out, err := shared.RunCommand("rsync", "--version")
lxd/storage/backend_lxd.go:             out, err := exec.Command("virt-v2v-in-place", "--version").CombinedOutput()
lxd/storage/drivers/driver_ceph.go:             out, err := shared.RunCommand("rbd", "--version")
lxd/storage/drivers/driver_cephfs.go:           out, err := shared.RunCommand("rbd", "--version")
lxd/storage/drivers/driver_cephobject.go:               out, err := shared.RunCommand("radosgw-admin", "--version")