[VTAdmin] Remove vtctld web link, improve local example (#15607)

[notfelineit]

This PR is the cherry-pick of #15607

Description

This PR:

• Removes the link to vtctld web, since we deprecated it in v17
• Improves vtadmin-up.sh by 1) outputting the variables that are being set and 2) shortening the wait time for cluster discovery
• Improves web/vtadmin/build.sh by 1) outputting the variables set and 2) when no hostname is set, uses the local machine's hostname

The last bullet point made it so that I could run ./101_initial_clusters.sh without any extra steps, and get vtadmin-web/vtadmin-api working:

[vitess-bot]

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