webadmin: Fix REST API URL in UI plugin infra

This patch ensures that UI plugin infrastructure
acquires REST API session using URL:

  /ovirt-engine/api/

(i.e. using appropriate base context path)

instead of current URL:

  /api/

which yields HTTP 404 response without redirect.

Change-Id: I5a538036f14a823ce831375ba3323fe2a0b7b7da
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>