TT#137760 Fix shellcheck issues

Fixes:

| In debian/ngcp-rtpengine-iptables-setup line 15:
| TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file=$CONFIG_FILE)
|                                                                  ^----------^ SC2086: Double quote to prevent globbing and word splitting.
|
| In el/rtpengine.init line 36:
| configfile=${CONFIG_FILE-/etc/rtpengine/rtpengine.conf}
| ^--------^ SC2034: configfile appears unused. Verify use (or export if used externally).
|
| In el/rtpengine.init line 38:
| TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file=$CONFIG_FILE)
|                                                                  ^----------^ SC2086: Double quote to prevent globbing and word splitting.

Those went unnoticed in commit 111b0a769c, but are failing
in github actions with shellcheck v0.7.2.

While at it, ensure that /etc/rtpengine/rtpengine.conf
is used as fallback, if CONFIG_FILE isn't defined.

Change-Id: I1c1948245a907bd6d299b242a9faff960d0608bc

debian/ngcp-rtpengine-iptables-setup

@@ -12,7 +12,7 @@ if [ -f "$DEFAULTS" ]; then
 fi
 
 MODPROBE_OPTIONS=""
-TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file=$CONFIG_FILE)
+TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file="${CONFIG_FILE-/etc/rtpengine/rtpengine.conf}")
 
 # Handle requested setuid/setgid.
 if ! test -z "$SET_USER"; then

el/ngcp-rtpengine-iptables-setup

@@ -11,7 +11,7 @@ if [ -f "$DEFAULTS" ]; then
   . "$DEFAULTS" || true
 fi
 
-TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file=$CONFIG_FILE)
+TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file="${CONFIG_FILE-/etc/rtpengine/rtpengine.conf}")
 MODPROBE_OPTIONS=""
 
 # Handle requested setuid/setgid.

el/rtpengine.init

@@ -33,9 +33,8 @@ rtpengine=/usr/sbin/rtpengine
 prog=rtpengine
 pidfile=${PIDFILE-/run/rtpengine.pid}
 lockfile=${LOCKFILE-/var/lock/subsys/rtpengine}
-configfile=${CONFIG_FILE-/etc/rtpengine/rtpengine.conf}
 cachefile=/var/lib/ngcp-rtpengine/rtpengine.cfg
-TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file=$CONFIG_FILE)
+TABLE=$(/usr/libexec/rtpengine/rtpengine-get-table --config-file="${CONFIG_FILE-/etc/rtpengine/rtpengine.conf}")
 PIDFILE=${pidfile}
 RETVAL=0
 OPTS=""