vyatta-cfg-dhcp-server: prefix host with shared-network-name

Host declarations in dhcpd.conf are global and not limited to the scope
they are defined in, so prefix the value given for a static mapping
with the shared-network-name to give a unique host entry in dhcpd.conf
for every static mapping.  The configuration file format and syntax
remains as it is, it is just the dhcpd.conf file produced as a result
that contains the prefix.

This patch also requires the patch for lib/DHCPServerOPMode.pm so the
prefix for this is stripped from the lease on "show dhcp server leases"
,at least for an output that stays consistent with the values in the
configuration file.

Bug #160 http://bugzilla.vyos.net/show_bug.cgi?id=160