cbbackup fails when CB_REST_USERNAME environmental variable is set

see CBSE-1492.
From customer
"cbbackup fails when CB_REST_USERNAME and CB_REST_PASSWORD are set. I have to unset them for cbbackup to run. Also cbbackup requires me to pass the credentials on the command line vs using the environment variables."

I have reproduced this issue, I had the same problem.. backups won't run unless the env vars are unset.
Also, there's no way to securely run backups, the username and password must be on the CLI.

./cbbackup couchbase://192.168.37.101:8091 ../backups/ -m full -u Administrator -p Foo -b default
error: SASL auth failed: 192.168.37.101:11210, user: default
vagrant@localhost:/opt/couchbase/bin$ env | grep CB_
CB_REST_USERNAME=Administrator
CB_REST_PASSWORD=Foo

$ unset CB_REST_USERNAME
$ unset CB_REST_PASSWORD
$ env | grep CB_
$ ./cbbackup couchbase://192.168.37.101:8091 ../backups/ -m full -u Administrator -p Foo -b default
################### 100.0% (1/estimated 1 msgs)
bucket: default, msgs transferred...
: total | last | per sec
byte : 68 | 68 | 116.1
done