When installing using some of the flags that persist in install, e.g
`linkerd install --ha`, and then doing `linkerd upgrade config` a nil
pointer error is thrown.
Fixes#3094
`newCmdUpgradeConfig()` was using passing `flags` as nil because
`linkerd upgrade config` doesn't expose any flags for the subcommand,
but turns out they're still needed down the call stack in
`setFlagsFromInstall` to reuse the flags persisted during install.
I also added a new unit test catching this.
Signed-off-by: Alejandro Pedraza <alejandro@buoyant.io>
Alejandro Pedraza