Refactor how we check if PostgreSQL should be prepared for migration

Having the same check in two places is error prone to not
updating both places with changes. Exactly as it happened
recently in 7c515c8.

Doing the check once and storing the result is better and will
also make it easier to change the check to something more complex
doing decisions on version differences and not hardcoded values.

Author: vpodzime

packaging/common/cfengine-hub/preinstall.sh

@@ -15,10 +15,16 @@ if is_upgrade; then
     fi
 fi
 
-BACKUP_DIR=$PREFIX/backup-before-postgres11-migration
-
 # If upgrading from a version below 3.13 that has PostgreSQL, and the data dir exists.
 if is_upgrade && egrep '^3\.([6-9]|1[012])\.' "$PREFIX/UPGRADED_FROM.txt" >/dev/null && [ -d "$PREFIX/state/pg/data" ]; then
+  alias migrating_postgres='true'
+else
+  alias migrating_postgres='false'
+fi
+
+BACKUP_DIR=$PREFIX/backup-before-postgres11-migration
+
+if migrating_postgres; then
   if [ -d "$BACKUP_DIR/data" ]; then
     cf_console echo "Old backup in $BACKUP_DIR already exists. Please remove before attempting upgrade."
     exit 1
@@ -125,7 +131,7 @@ if is_upgrade; then
   fi
 fi
 
-if is_upgrade && egrep '^3\.([6-9]|1[01])\.' "$PREFIX/UPGRADED_FROM.txt" >/dev/null && [ -d "$PREFIX/state/pg/data" ]; then
+if migrating_postgres; then
   cf_console echo "Moving old data and copying old binaries to $BACKUP_DIR"
   # Now that PostgreSQL is shut down, move the old data out of the way.
   mkdir -p "$BACKUP_DIR/lib"