Auth: overriding auth methods

[kayrus]

Initial issue: https://github.com/terraform-providers/terraform-provider-openstack/issues/638

There is no way to explicitly set the auth method. v2auth and v3auth funcs override already defined auth opts from the environment variables.

There are multiple solutions to resolve this issue.

• Change EnvPrefix type from string to *string and allow defining an empty prefix. This will allow to specify options from env variables with no prefix.
• Move out condiotions to the top level. This will allow to override password auth with app creds, but not vice versa.
• Introduce EnvOverride bool option into ClientOpts.