packaging: setup: using engine-common in websocketproxy plugins

Loosen up engine-common requirements in order to use it
from wsp plugins.

Change-Id: I7cb1245105f913f0c82fb3b909f14584053e8a0f
Bug-Url: https://bugzilla.redhat.com/1125188
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>