Rename `fact` to `fact_on` for uniformity

The DSL method `fact` was recently added, but most dsl methods take the
form of `<method>_on` for multi-node functions and `<method>` for
running on a default node. This makes `fact_on()` run on designated
nodes, and `fact()` run on the default node.
Merge pull request #78 from hunner/fact_on

Rename `fact` to `fact_on` for uniformity
Destroy vagrant hosts only if Vagrantfile exists

The new `vagrant destroy --force` run during the vagrant hypervisors
`provision` method added in #48 seems to fail if no `Vagrantfile` has
ever been created for a nodeset. This fixes that.
Merge pull request #77 from hunner/destroy_nothing

Destroy vagrant hosts only if Vagrantfile exists