logging: allow passing logger to `NewRunner`

[jrasell]

Ref code:

consul-template/manager/runner.go

Line 209 in e60eab9

func NewRunner(config *config.Config, dry bool) (*Runner, error) {

Nomad utilizes consul-template as a library and it would be great if we could optionally pass a logger when setting up a new runner. This could avoid some of the intercept overhead from our template runner associated to logging which is currently required to get CT logs showing up within the Nomad client logging.

Related: #1522