ctdb-scripts: Avoid shellcheck SC1117: Backslash is literal in "..."

ctdb-scripts: Avoid shellcheck SC1117: Backslash is literal in "..."

This warning (apparently new in shellcheck 0.4.7) only applies to
double-quoted strings.  Change affected constant strings to use
single-quotes.  In the one example that contains a variable expansion
escape the backslash as recommended.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>