ctdb-scripts: Quote some variable expansions

ctdb-scripts: Quote some variable expansions

This avoids relevant shellcheck warnings.  This is most of the
shellcheck low hanging fruit in the non-test code.  Many of the other
warnings produced by shellcheck are either false positives, are
non-trivial to fix or a fix may result in worse code.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Amitay Isaacs <amitay@samba.org>
Autobuild-Date(master): Wed Jul  6 08:15:49 CEST 2016 on sn-devel-144