VaultSigner test unexpectedly does not depend on environment variables

[lukpueh]

The HashicorpVault client used by VaultSigner to import_ a public key or sign, needs a vault address and a token. We expect users to provide both via specific envvars known to the client, and so do the tests:

securesystemslib/tox.ini

Lines 122 to 123 in 5e734e4

	VAULT_ADDR = http://localhost:8200
	VAULT_TOKEN = test-root-token

But it turns out that the tests work even if the envvars are not available to VaultSigner, because:

• above VAULT_ADDR is the default address, which the client falls back to, if the env var is not set, and
• vault server -dev (in init-vault.sh) secretly writes the token to a ~/.vault-token file, which the client falls back to, if the env var is not set

This is not necessarily an issue but a bit surprising, so I think a note in VaultSigner code doc and tox.ini might be fix enough.

Alternatively, we can set VAULT_ADDR to a non-default port and start the test server like so:

vault server -dev \
    -dev-listen-address="${VAULT_ADDR#http://}" \  # unlike VAULT_ADDR this option does not want the protocol prefix
    -dev-root-token-id="${VAULT_TOKEN}" \
    -dev-no-store-token \
    &