No longer use existing installs by default and change commands accordingly

Changes ignore-existing flag to use-existing and inverts default behavior.
Also adds this to docs.

Refs #5336

Change-Id: I1f3f68160673321e92307c9aacc7ff361b1d1a48
diff --git a/util/pkgdep/common.sh b/util/pkgdep/common.sh
index 60749ef..62ab1ee 100644
--- a/util/pkgdep/common.sh
+++ b/util/pkgdep/common.sh
@@ -127,7 +127,7 @@
 
 # return whether dep is installed
 dep_exists() {
-  if [[ $IGNORE_EXISTING -eq 1 ]]; then
+  if [[ $USE_EXISTING -eq 0 ]]; then
     return 2
   fi
   local DETECT="${DEP_DETECT[$1]}"