Support for custom mountpoint on login

[msessa]

In Vault it's possible to mount an authentication backend on a non-standard path (example: userpass backend mounted on auth/employees).

This PR adds support for specifying a custom path in the login settings dialog.

In addition a custom path can be pre-configured by a system administrator with the VAULT_AUTH_BACKEND_PATH environment variable.

[djenriquez]

@msessa-cotd

Do you know how often these paths are overridden? The path variable seems like it could be something that would confuse a user who doesn't know that ability exists. Would it make sense for this field to be something that is collapsed by default and only appears if a check box or something is clicked?

[msessa]

You might be right. I could be confusing.

I tend to assume most vault users are power users and pre-configuration using environment variables is the way to bridge the gap.

Anyway, I'll implement your suggestion.

[msessa]

Ok I've nested the textfield in a collapsed Card component.

[djenriquez]

LGTM, thanks for adding the card.