(QENG-833) Fixes use of Beaker logger in retry_command.

Looks like the Beaker DSL always assumes a method, `logger`, that returns a
Logger object--assuming that the namespace loading the DSL modules has a logger
object available as `@logger` breaks beaker-rspec.

Signed-off-by: Wayne <wayne@puppetlabs.com>
Merge pull request #317 from waynr/bugfix/retry-command-should-use-logger-method

(MODULES-1168) Fixes use of Beaker logger in retry_command.