_frontend/main.py: Use click.INT type for bst workspace --source

Thomas Chetwinrequested to merge
bloomberg/bst/buildstream:workspace-source-integer into master

bst workspace {open,close,reset} <bst> --source <index> commands currently give an indexing error. The python click library needs to be told that the expected argument is of type integer.

This patch also makes use of metavar to display a placeholder in the help prompt, like:

-s, --source INDEX The source to create a workspace for. Projects with one source may omit this

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