toaster: use knotty when working with bitbake server

toaster: use knotty when working with bitbake server

Using empty BITBAKE_UI environment variable causes bitbake server
to fail with the error:
    FATAL: Unable to import extension module "" from bb.ui
           Valid extension modules: knotty or toasterui\n'

Used BITBAKE_UI="knotty" when starting and stoping bitbake
server to solve above issue.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>