dashboard: Fix merge parameters

When there is a duplicate key, the key in the rightmost hash will "win."
Currently extra_params default values can never be overriden by
vhost_extra_params if we have duplicate key.
This patch allows it.

https://forge.puppetlabs.com/puppetlabs/stdlib#merge

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>