(QENG-270) Added ntp_server as an option for host config

Before this, you couldn't specify an ntp server, so if you used the timesync option, it would only use the hard-coded value 'pool.ntp.org'.
This was a problem because occasionally that service couldn't be reached.  In order to fix this, we've allowed people to specify a server, so that they can get more reliable ntp services, and not see failures based on being able to reach one.
Merge pull request #486 from kevpl/qeng270_ntp_addoption

(QENG-270) Added ntp_server as an option for host config