Add RUN_EXERCISES to exercise.sh.

The exercise.sh accepted a SKIP_EXERCISES var for limiting which
exercises run.  Sometimes I only want to run one of this.  This new
variable make that much easier than having to list every other exercise
as one that should be skipped.

Change-Id: I21d3cc6ead677d25f7697c6cc1fd690cbbbfbdd7
Deprecate use of fixed_range flag in nova conf

Since a3786b4b8aa47cf19b9dc2babb4df85f578a1280 merged into nova we
now can default to setting fixed_range in nova.conf to "". This will
cause nova to do automatic NAT set up based on the networks
configured rather than relying on the user to set fixed_range to the
largest subnet that includes the networks they've configured.

Leave the stack.sh default value of $FIXED_RANGE alone because it is
used for creating the networks, etc.

Change-Id: Iad13588af3dcf22ce81823de301b73b05cf2aae9
Extract HOST_IP default process to a function

This process is required by Grenade also...

Change-Id: I263a4351e70b9cfb0965e57e518fe5fd377f4b43
Merge "Add RUN_EXERCISES to exercise.sh."
Merge "Extract HOST_IP default process to a function"
Merge "Deprecate use of fixed_range flag in nova conf"