vcd-cli login supported user types are not explicitly stated

[troshlyak]

I was not able to see any reference in both the help messages and the documentation that would explicitly mention what type of accounts are supported through the vcd login command. As currently it seems that SAML is not supported, users attempting to login in organizations configured with SAML, fail to do this but assume that they should. Maybe the help message should clarify that only Local/LDAP accounts can be used with the vcd login command, to remove this confusion.