Allow indefinite lease times to be set

See https://serverfault.com/questions/505300/isc-dhcp-infinite-lease-time
Fixes voxpupuli#219

Author: alexjfisher

manifests/init.pp

@@ -26,8 +26,8 @@
   Optional[String[1]] $ipxe_filename                               = undef,
   Optional[String[1]] $ipxe_bootstrap                              = undef,
   Dhcp::Syslogfacility $logfacility                                = 'daemon',
-  Integer[1] $default_lease_time                                   = 43200,
-  Integer[1] $max_lease_time                                       = 86400,
+  Integer[-1] $default_lease_time                                  = 43200,
+  Integer[-1] $max_lease_time                                      = 86400,
   Stdlib::Ensure::Service $service_ensure                          = 'running',
   Variant[String,Array[String[1]]] $globaloptions                  = '',
   Optional[Stdlib::Port] $omapi_port                               = undef,