Checking credentials only when needed

[danielhuppmann]

Working on the ixmp4-pyam integration, I noticed that ixmp4 immediately (tries to) connect to the IIASA-ScSe-Manager, resulting in a log output like

This can be confusing to pyam-only-users, so this PR changes the order so that the credentials are only loaded when needed.

[danielhuppmann]

One more change added: I removed the warning "No default credentials provided." because

1. It is not necessary to provide credentials to use ixmp4, hence no warning necessary.
2. There is currently no way (and no need?) to use non-default credentials anyway, hence the message is not necessary.

Note that there will still be a log message

ixmp4.conf.auth - INFO: Connecting to service anonymously and without credentials.

when connecting as anonymous user, which is sufficient for a normal user.

[meksor]

Sounds good, that was more the point of the message anyway! I think I might implement a way to use different credentials on the dev manager instance for example, hence the whole 'default'-and-such terminology.