Fix adding and deleting keys by using named variables

Initial positional parameters have already been parsed and stored in
named variables early, so use them instead of positional parameters

Some wrong conditions have been fixed, and this fixes
cedricziel#30.

commands

@@ -131,7 +131,7 @@ EOF
 
 add_shared_key() {
   check_install
-  if [[ ! -n "$2" ]]; then
+  if [[ -n "$SHARED_KEY_TO_ADD" ]]; then
     echo $SHARED_KEY_TO_ADD >> "$SHARED_HOSTKEYS_FILE"
     echo "Added $SHARED_KEY_TO_ADD to the list of shared hostkeys"
   else
@@ -141,7 +141,7 @@ add_shared_key() {
 
 delete_shared_keys() {
   check_install
-  if [[ ! -n "$2" ]]; then
+  if [[ -n "$HOSTNAME_TO_REMOVE" ]]; then
     ssh-keygen -f "$SHARED_HOSTKEYS_FILE" -R "$HOSTNAME_TO_REMOVE"
     rm "$SHARED_HOSTKEYS_FOLDER/known_hosts.old"
     echo "Deleted hostkey for $HOSTNAME_TO_REMOVE as well as the backup."
@@ -165,7 +165,7 @@ add_app_key() {
   check_app
   check_exists
   check_install_app
-  if [[ ! -n "$3" ]]; then
+  if [[ -n "$SHARED_KEY_TO_ADD_APP" ]]; then
     echo $SHARED_KEY_TO_ADD_APP >> "$APP_SPECIFIC_HOSTKEYS_FILE"
     echo "Added $SHARED_KEY_TO_ADD_APP to the list of shared hostkeys"
   else
@@ -189,7 +189,7 @@ delete_app_keys() {
   check_app
   check_exists
   check_install_app
-  if [[ -n "$3" ]]; then
+  if [[ -n "$HOSTNAME_TO_REMOVE_APP" ]]; then
     ssh-keygen -f "$APP_SPECIFIC_HOSTKEYS_FILE" -R "$HOSTNAME_TO_REMOVE_APP"
     rm "$APP_SPECIFIC_HOSTKEYS_FOLDER/known_hosts.old"
     echo "Deleted hostkey for $HOSTNAME_TO_REMOVE as well as the backup."
@@ -227,11 +227,11 @@ case "$1" in
     ;;
 
   hostkeys:shared:add)
-    add_shared_key "$@"
+    add_shared_key
     ;;
 
   hostkeys:shared:delete)
-    delete_shared_keys "$@"
+    delete_shared_keys
     ;;
 
   hostkeys:shared:autoadd)
@@ -243,11 +243,11 @@ case "$1" in
     ;;
 
   hostkeys:app:add)
-    add_app_key "$@"
+    add_app_key
     ;;
 
   hostkeys:app:delete)
-    delete_app_keys "$@"
+    delete_app_keys
     ;;
 
   hostkeys:app:autoadd)