Update values.yaml to handle tlsDisable=false and update developer documentation

Summary

Currently there are a few cumbersome changes that are needed to be made in order to have Vault work with a PASSTHROUGH gateway (i.e. TLS termination occurs within the Vault container itself, not the istio sidecar). We should look to make it possible to dictate Vault configuration based on the state of a few key values:

1. global.tlsDisable
2. istio.vault.tls.key
3. istio.vault.tls.cert

If the three values above are truthy, we should set Vault configuration for ha, standalone, and raft use cases accordingly, to serve up the provided certificates. This involves adding the following lines to the config block for each of the three deployment strategies:

tls_disable = 0
tls_key_file = "/vault/tls/tls.key"
tls_cert_file = "/vault/tls/tls.crt"

Additionally, the developer documentation should be updated to give better guidance for how BB developers can set up Vault to work with a PASSTHROUGH gateway, as the instructions are currently innaccurate/incomplete.