Fixed error on workers tab.

Fixed undefined method `[]' for #<VMDB::Config:0x00000006f06a80> when editing any field on workers tab. We cannot set auth from new[:authentication], @edit[:new] is a config object on the Workers tab. This broke with changes for https://github.com/ManageIQ/manageiq/pull/954
Merge pull request #1188 from h-kataria/workers_tab_edit_fix

Fixed error on workers tab.