Enable -timeout option when detecting OS

[knqyf263]

What did you implement:

I got the following error.

$ vuls configtest
[Mar 31 20:12:18]  INFO [localhost] Validating config...
[Mar 31 20:12:18]  INFO [localhost] Detecting Server/Container OS...
[Mar 31 20:12:18]  INFO [localhost] Detecting OS of servers...
[Mar 31 20:12:28] ERROR [localhost] Timed out while detecting servers
[Mar 31 20:12:28] ERROR [localhost] (1/1) Timed out: ubuntu1404
[Mar 31 20:12:28] ERROR [localhost] Failed to init servers: No scannable servers
exit status 1

The timeout for detecting server OS is hard coded to 30 seconds.
https://github.com/future-architect/vuls/blob/master/scan/serverapi.go#L156
https://github.com/future-architect/vuls/blob/master/scan/serverapi.go#L218

I fixed this.

How did you implement it:

For configtest, -timeout option has already implemented, so I used it.
For scan, I added -timeout option.

How can we verify it:

$ vuls configtest -timeout 120
$ vuls scan -timeout 120

Todos:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[kotakanbe]

LGTM

[kotakanbe]

Many Thanks!