Use FHS-compliant default template location

/opt/stack is a very OpenStack-specific location to use as a
default for a project that is intended to be usable elsewhere.
This changes the default template directory to reside in
/usr/libexec and deprecates the /opt/stack location, but still
allows the use of /opt/stack so existing users won't be adversely
affected.

Change-Id: I3b7d356985a2c213303d52e36aec34e350f9b553
Merge "Use FHS-compliant default template location"