Delete the lb configuration files during cleanup

The issue is seen because boost::filesystem::remove
is used to delete the lb pool configuration directory.
Since the directory is not empty, it is failing.

This patch fixes the issue by calling
boost::filesystem::remove_all instead of delete
the files if the directory is not empty.

Change-Id: I0b22c985aacee9173f1537f029fdc51c6db0324b
Closes-bug: #1399241