Add repeat option to dhcp check

repeat option was disabled when there was no tests for dhcp checker

Repeat logic:

send discover -> wait -> send -> wait

Keep in mind that it does not stop when first dhcp offer is received,
so for example if 3 repeat with timeout 10 will be set - it will run atleast for 30 seconds


It does not affect current deployment because of usage repeat=1
in net_probe.rb

Change-Id: I9deec4d9bf9ef648518c678218e2460d3170bec3
Related-Bug: #1320834
Related-Bug: #1317525
Merge "Add repeat option to dhcp check"