UX: fixed some ux issue

- fixed english grammar
- when checking again engine liveliness avoid
  to ask to install engine
- uniformed Yes/No case
- disambigued some dialog messages
- clarified some user instructions

Change-Id: Iaf6caef215f94d8d400eeb9c2ce6b38c38a62ab3
Reported-By: Greg Padgett <gpadgett@redhat.com>
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>