Description of problem:
cleanup algorithm 

Version-Release number of selected component (if applicable):
snapper-0.1.7-6.el7.*

How reproducible:

have following lines in /etc/snapper/configs/<config_name> file:

# cleanup empty pre-post-pairs
EMPTY_PRE_POST_CLEANUP="yes"
# limits for empty pre-post-pair cleanup
EMPTY_PRE_POST_MIN_AGE="10" (or other low enough value in secs)


Steps to Reproduce:
1. create empty pre-post snapshot couples: snapper -c <config_name> create -c empty-pre-post --command "echo idling"
2. wait for $EMPTY_PRE_POST_MIN_AGE secs
3. snapper -c <config_name> cleanup empty-pre-post

Actual results:
[root@localhost ~]# snapper -c testsuite cleanup empty-pre-post
Config is in use.

Expected results:
Empty pre post snapshots are removed.