Add deploy start behavior option

[ryan-lane]

This option should be a list so that users can specify multiple start behaviors. There's a specific initial use case:

It's possible to do git changes before a deployment is started. To prevent accidental changes creeping into a deployment, we should have an option 'reset-to-current-tag', which would reset the repository to the state of the current deployed tag when the deployment starts. This should always fetch the remote tags to ensure the reset is occuring to the latest tag, in case the users are not using a deployment server.