Use FHS-compliant default base directory

/opt/stack as the default base directory is not very friendly to
non-OpenStack users, so this changes the default base directory
to reside in /usr/libexec.  The /opt/stack path will continue to
work for now so any existing users will not be broken by the change.

Also changes the directory name from os-config-refresh to
os-refresh-config to better align with the os-apply-config naming.

Change-Id: I98e93d91685ae25ae74a5470483a1cc8b97da4e5
Merge "Use FHS-compliant default base directory"