systemd: add lvm2 activation generator

The lvm2 activation generator generates systemd units conditionally
based on the global/use_lvmetad lvm.conf setting.

If use_lvmetad=0, the lvm2-activation-early.service and lvm2-activation.service
units will be generated. These units are responsible for direct volume activation
by calling "vgchange -aay --sysinit" (this is actually the original on-boot
activation as it was used before). If use_lvmetad=1, no units will be generated
as we're relying on autoactivation.

Important thing to note is that the lvm2-activation units normally bring
in the udev-settle ("storage-wait") service that waits for udev to settle
(with block devices). We don't need this if lvmetad is used in conjunction
with autoactivation feature... but systemd units can't be enabled or disabled
(or dependencies added/removed) dynamically based on external configuration.

Therefore, we need the unit generator which adds support for such situations:
the units as a whole either exist or not based on the external configuration.

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