Remove unneeded fs.file-max setting from sysctl tuning file

The kernel (since at least some 2.6 release) automatically sets a sane value
for fs.file-max based on the amount of memory installed in that system. That
value (around 100 file descriptors per 1MB of RAM) is usually much higher than
the value (65535) this template was trying to set.
Allow using the serial number for defining the bus order

This introduces a new optional attribute "serial_number" in the interface_maps
that can be use as additional matching criteria for defining the interface
order for a certain node. (This is helpful e.g. if two nodes with the same
Product Name need to have different interface_map defined)
Handle empty network definitions in recipe

This just makes the code more solid.
Fix file permissions

Remove unnecessary executable bit by running:

    find crowbar_framework/ -type f -perm /g=x | xargs chmod -x
Merge pull request #191 from rhafer/fix/10gbe-tuning

Remove unneeded fs.file-max setting from sysctl tuning file
Merge pull request #193 from vuntz/avoid-crash

Handle empty network definitions in recipe
Merge pull request #192 from rhafer/pull-req-release-pebbles-master-ead8713cdc2ce28d97bab36901b28238e32bb08e

Allow using the serial number of a node as an additional matching criteria for the bus_order [3/3]