packaging: setup: bridge name in hosted-engine.conf

Added bridge key in hosted-engine.conf allowing the
HA agent to parse it in an easy way.
The value is now assigned from environment variable
instead of being hardcoded in the template

Bug-Url: https://bugzilla.redhat.com/1010866
Change-Id: I698d6e7609ef20dff0426b07028ca3391e82a0d7
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>