(maint) Honor TEST or TESTS and timesync when provisioning

Harness indicated you should set TESTS to override beaker --tests, but
was only looking at ENV['TEST'].  Accepts either TEST or TESTS now.

Also sets :timesync => true in base beaker options, replacing the :ntp
setting which can only be passed as a command line flag.  So beaker will
now perform ntpdate or equivalent on new hosts.