Increase settings for dnsmasq and sysctl

* Make a new variable dhcp_lease_max. It increases the number of
  available leases from 1000 to 1800. It allows to provision nodes on
  scale, when Debian Installer or Anaconda looses IP in the middle of
  install.
* Make a new variable lease_time. It increases the default lease size
  to 120m, up from the default 60m.
* Add cache-size to dnsmasq template. dnsmasq will keep more entries in
  case.
* Increased neighbour table on master node to keep more ARP requests
  that come in parallel once deployment is started. This change also
  removes unneed broadcast traffic. New values are:
  net.ipv4.neigh.default.gc_thresh1 = 256
  net.ipv4.neigh.default.gc_thresh2 = 1024
  net.ipv4.neigh.default.gc_thresh3 = 2048
* Fix linting

Related-Bug: #1376680
Related-Bug: #1379917
Related-Bug: #1381997
blueprint 100-nodes-support

Change-Id: I4da8070143e401f7a9246e72eda35e601b8c6386
Merge "Increase settings for dnsmasq and sysctl" into stable/5.1