Do not use return in ruby_block

This is not needed, and this creates the following error:

LocalJumpError
--------------
unexpected return

Cookbook Trace:
---------------
/var/chef/cache/cookbooks/ipmi/recipes/ipmi-discover.rb:66:in `from_file'

Note that there's nothing after the ruby_block anyway, so the intent of
return is still preserved.
Do not fail in ipmi-configure if calls to ipmitool fail

I guess it's buggy hardware, but we're seeing calls to
    ipmitool lan set 1 defgw ipaddr
fail on some hardware. Unfortunately, this happens on
hardware-installing and this means that everything after ipmi-configure
will not run. Including the bootdisk finder in provisioner.

So instead of using a bash block, we use a ruby_block where we can see
the exit value of ipmitool.
Merge pull request #43 from vuntz/no-return-in-ruby_block

Do not use return in ruby_block
Merge pull request #44 from vuntz/ipmi-configure-no-fail

Do not fail in ipmi-configure if calls to ipmitool fail