docs/install-instr: Add target specific cfg sect.

docs/install-instr: Add target specific cfg sect.

Fuel 8.0 adds support for setting up "Offloading Modes" for
target nodes.

Document how this features should be used (and why/when it's
necessary), and also its common limitation of not being applied
during Verify Networks step (see related Fuel bug report [1]).

While at it, fix minor reference to branch in git tag checkout
instructions, as we'd be in detached HEAD and not on a new branch.

[1] https://jira.opnfv.org/browse/FUEL-134

[Alexandru.Avadanii@enea.com]
Only minor edits and prepared for upstreaming, this is mostly
based on previous work from Florin.

Change-Id: If7a8ca44541903dbeb6db36bc64fe56b132165cb
Signed-off-by: Florin Dumitrascu <florin.dumitrascu@enea.com>
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>