Use vault_api_addr to set VAULT_ADDR in .bashrc

[akerouanton]

Prior to this PR, vault_api_addr and vault_cluster_addr were defined using the IPv4 address of the default interface of the host. Moreover, a variable named vault_hostname was used to run the reachability check but was undocumented.

This change documents vault_hostname and leverages it to set the value of vault_api_addr and vault_cluster_addr when it's available. Otherwise, these vars are defined as before.

Moreover, vault_api_addr is now used to set the URL used by the reachability check and to set the value of VAULT_ADDR in .bashrc. Before that change, these URLs were define by concatenating the value of vault_addr and vault_port together. vault_addr was defined as being either 127.0.0.1 (when vault_address was 0.0.0.0) or the value of vault_address. However, vault_address is used to define the IP address Vault should bind to. Although, by default, they were defined to be exactly the same as vault_api_addr default value, when overriding that param (for instance to put an hostname instead of the IP address), the value of VAULT_ADDR (and the URL used by the reachability check) could be wrong in some cases (eg. when using TLS certs with no IP: 127.0.0.1 SAN). Instead of adding a new var to override the value of VAULT_ADDR, this change reuses the value of vault_api_addr, which is now defined through vault_hostname param (when provided).

[akerouanton]

ansible-lint breaks on vault_api_addr being too long but I don't know how to fix it. Writing it on multiple lines with |-2 still adds a space between each parts. I'd need some help to fix this issue.