(QA-805) Add timestamp to on(host) calls

Previously there was no timestamp, which makes it hard to correlate
beaker commands with logs on the SUT.

    lw5bhs6m2zu4ysb (centos5-64-1) 14:21:44$  which curl
    /usr/bin/curl

    lw5bhs6m2zu4ysb (centos5-64-1) executed in 5.99 seconds
Merge pull request #139 from branan/command_timestamp

(QA-805) Add timestamp to on(host) calls