toolbox: Improve toolbox welcome message

This dynamically [1] lists the possible configuration files we could choose from. No need to look them up.

[0] Possible output from the current minikube toolbox (with the patch applied)

swh@swh-toolbox-7c974cfc8b-k27xg:~$ source .bashrc
SWH_CONFIG_FILENAME variable is not set!

 This variable must be defined according to your use case (e.g. .
 scheduler, storage, vault, ...). You must define it by yourself.

 For example, use one of the following:

export SWH_CONFIG_FILENAME=/etc/swh/config-indexer-storage.yml
export SWH_CONFIG_FILENAME=/etc/swh/config-storage.yml
export SWH_CONFIG_FILENAME=/etc/swh/config-webhooks.yml
export SWH_CONFIG_FILENAME=/etc/swh/config-vault.yml
export SWH_CONFIG_FILENAME=/etc/swh/config-scheduler.yml
export SWH_CONFIG_FILENAME=/etc/swh/config-scrubber-storage.yml

[1] Depending on what's currently declared in the toolbox we connect to