[Installer] Fixed sed pattern which resulted in unexpected matches for 'nginx'

Fixes issue #824

(cherry picked from commit 0facc01420efcf3bc5b7e3dcf50a5673e2d1b234)

install/hst-install-ubuntu.sh

@@ -713,7 +713,8 @@ fi
 # Excluding packages
 software=$(echo "$software" | sed -e "s/apache2.2-common//")
 if [ "$nginx" = 'no'  ]; then
-    software=$(echo "$software" | sed -e "s/\bnginx\b/ /")
+    software=$(echo "$software" | sed -r -e 's/(\s|^)nginx(\s|$)/ /')
+
 fi
 if [ "$apache" = 'no' ]; then
     software=$(echo "$software" | sed -e "s/apache2 //")