Added TestSelectUIDPasswordProvider to login flow tests.
Added TestSelectUIDPasswordProvider to login flow tests.

This tests the code added to detect when the 'dcos-users' provider is
available and select it even if multiple providers are available.

Co-authored-by: Armand Grillet <25d46a51a6c82fc53ca69a613ab205b38a8d5e15@outlook.com>
Co-authored-by: Bilal Amarni <35032ed22b4c97b634b8ca4b28bb6ea86a4ac100@gmail.com>

dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli