Low: LVM: Detect duplicate resources with the same volgrpname name when ...

...using exclusive activation with tags.

For exclusive activation with tags, we already detect
if the resource is a clone and prevent the LVM resource
from activating.  This check goes a step further and
detects if there is another primitive attempting to access
the same volume group in pacemaker's configuraiton. This
check is ignored if pacemaker is not in use, allowing this
agent to continue to be portable to other cluster managers
in the future.

This patch also re-words the 'exclusive' option's documentation
in order to clarify how volume_list filtering works.

Resolves: rhbz#984054