tools: Doc fixes of provider type in engine-manage-domains

Fixes provider type description in man page and help for
engine-manage-domains.

Change-Id: I013547823809e1cec8025f3bcbcfa7577939b76f
Bug-Url: https://bugzilla.redhat.com/1056999
Signed-off-by: Martin Perina <mperina@redhat.com>