Make it possible to pull and push into online repositories with http
authorization.
Changes:
* Separated dialogs for entering username and password are merge
together
* Do not show dialog for local repos
* Prohibit passwords for ssh scheme

Task-number: QTCREATORBUG-5104
Change-Id: I22fe3d778bb43d25ccd3ee423f98b22dd2019c24
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>