dmeventd: lvm2 plugin uses envvar registry

dmeventd: lvm2 plugin uses envvar registry

Thin plugin started to use configuble setting to allow to configure
usage of external scripts - however to read this value it needed to
execute internal command as dmeventd itself has no access to lvm.conf
and the API for dmeventd plugin has been kept stable.

The call of command itself was not normally 'a big issue' until users
started to use higher number of monitored LVs and execution of command
got stuck because other monitored resource already started to execute
some other lvm2 command and become blocked waiting on VG lock.

This scenario revealed necesity to somehow avoid calling lvm2 command
during resource registration - but this requires bigger changes - so
meanwhile this patch tries to minimize the possibility to hit this race
by obtaining any configurable setting just once - such patch is small
and covers majority of problem - yet better solution needs to be
introduced likely with bigger rework of dmeventd.

TODO: avoid blocking registration of resource with execution of lvm2
commands since those can get stuck waiting on mutexes.