Don't allow username/password auth

[justaugustus]

No description provided.

[lgecse]

@justaugustus What we do now is username + APIToken basic auth. Should we remove this capability completely from the tool?

My recommendation is to keep it in the tool, but rename the config at least from jira-password to jira-apitoken. We are not allowing already to use the tool with jira username and password line 107 in jira.go

[rappizs]

Should we remove this capability completely from the tool?

I don't know how we could remove this auth method since other than this Jira only supports OAuth, which although is more secure, isn't really useful if we want to run the tool in an automatised way.

At the moment we support this basic auth (Basic auth for REST APIs) and OAuth 1.0a which is deprecated and should be replaced, see #31 (OAuth 1.0a for REST APIs (Legacy)).

I agree with @lgecse that we should keep this method and rename the jira-pass config value, since it's not really a password but an API token.