ws: Remove the warning when adding a server with a blank user name

Fixes #941
Closes #958
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: gives user domain admininistrator hint text

After filling in a correct Domain Address, instead of overwriting
the Domain Administrator Name there will be suggested text put
there. This suggested text will put the same values in the text box but in
suggested text form. Furthermore, the text indicates to the user that
it is an example because it might not work for the user's domain.

Fixes #937
Closes #955
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: removes "options" text in join dialog

Fixes #952
Closes #954
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Add input space validation when adding a server

Add input validation and visual style when error occurs.
Remove get_focus and lose_focus functions.
Add some restrictions for empty host name in case "Next" button is enabled accidentally.

Fixes #943