Add support of shared-network option about puppet-dhcp

[CelestianGC]

Indeed. This is a option critical to our use.

[ekohl]

A patch for this is always welcome. If needed, we're happy to help you write it and get it merged.

[sircubbi]

I created a small patch to group several dhcp::pool and dhcp::pool6 definitions into shared-network-segments (see #261).

To use this both defines have an additional parameter sharednetwork which accepts a string to which shared-network-segment that pool belongs too.

Then each segment is created by the define dhcp::sharednetwork (the title should be identical to the parameter from above) which optionally accepts an array of global parameter to define defaults for all pools in it. A list of segments can also be specified via hiera/classparameter as dhcp::sharednetworks.