Introduce lvm2_install target.

Introduce lvm2_install target.

Buildsystem support device-mapper only install,
but generic install tagret includes both dm+lvm2.

For distribution which uses separate install_device-mapper,
there is no way how to install lvm2 only
(so after installing lvm2 for packaging purposes
built system must remove installed device-mapper files).

Fix it by allowing lvm2_install target, similarily like
install_cluster for clvmd.

(install = install_device-mapper + install_lvm2)