Fixes bad validMountMode check

Needed to check if the mode was invalid and return error, not valid and
return error.

This didn't get picked up because the existing integration-cli tests
were all either expecting errors when a valid mode was passed in (e.g.
"ro" passed in, we expected an error because it was testing write).  So
modified a test which was testing for "rw" to actually pass in "rw"
instead of assuming the "rw"

ping @crosbymichael @vieux 
This needs to be merged before release!