Make X-RUCIO-ACCOUNT header optional in auth server

[maany]

Motivation

In the rucio server, /auth/userpass endpoint has a hard requirement for the account header.

rucio/lib/rucio/web/rest/flaskapi/v1/auth.py

Lines 110 to 111 in 5c665be

	if not account or not username or not password:
	return generate_http_error_flask(401, CannotAuthenticate.__name__, 'Cannot authenticate without passing all required arguments', headers=headers)

This complicates the login workflow for the new webui. The login page asks the user for the identity i.e. username' and password` but if the user does not specify the account, the login will fail.

In order to make the account field optional, this endpoint must be modified

Modification

• if the header is present, use that account
• if the header is not present, list the accounts associated with the identity.
  • if 0 accounts are found, raise appropriate error
  • if a single account is found, use that
  • if multiple accounts are found
    • if default account is specified, use that
    • otherwise, return a list of accounts that the UI can render and ask the person to pick one explicitly

This must be done in a way that other endpoints can re-use the logic