(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.

Because the debugging output for commands was getting long, I've also
changed it to a two-line format.

    14:21:44 lw5bhs6m2zu4ysb (centos5-64-1)
    $  which curl
    /usr/bin/curl

    14:21:50 lw5bhs6m2zu4ysb (centos5-64-1)
    executed in 5.99 seconds