configurable dns search domains

Add a --dns-search parameter and a DnsSearch
configuration field for specifying dns search
domains.

Docker-DCO-1.1-Signed-off-by: Daniel Norberg <daniel.norberg@gmail.com> (github: danielnorberg)
fix typo in documentation

Docker-DCO-1.1-Signed-off-by: Daniel Norberg <daniel.norberg@gmail.com> (github: danielnorberg)
variable declaration cleanup

Docker-DCO-1.1-Signed-off-by: Daniel Norberg <daniel.norberg@gmail.com> (github: danielnorberg)
Merge pull request #3774 from danielnorberg/dns-search-domains

configurable dns search domains