Re-introduce timeout option for lava-slave
--timeout option was removed from lava-slave command line recently along with re-factoring lava-slave code via [https://review.linaro.org/#/c/23639]
Re-introduce this option to give more control on the slave side, since the ping intervals should be configurable from the workers.
NOTE: This was discussed in LAVA Design Discussion on 29^th^ May, 2018
Migrated from https://projects.linaro.org/browse/LAVA-1339