add the possibility to read dsl from a file

This allows to store the DSL under source control with all the benefits
that this can bring. This feature is only available when the "Flow run
needs a workspace" option is enabled.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Merge pull request #47 from 6WIND/dslfile

add the possibility to read dsl from a file