LVM-activate: put all validations into lvm_validate

Checking anything before reaching monitor operation is wrong.
The monitor op is always called by pacemaker to know the status
of resource. If the monitor op fails, the stop op will be called
by pacemaker to try to make sure the resource is stopped. In
LVM-activate, the vgname, activation_mode, vg_access-mode are
checked very early before any real operation. In case the vgnane
is not specified, the monitor and stop operations will fail even
thought no resource is actually running.

Signed-off-by: Eric Ren <zren@suse.com>