Unable to re-save newly created workflows / actions

[Kami]

If I use st2flow to create a new workflow, initial click on "save" button will work fine, but subsequent ones will fail with "Error saving workflow.: Tried to save duplicate unique keys (E11000 duplicate key error collection: st2.action_d_b index: uid_1 dup key: { : "action:fixtures:dsada1" })".

I believe that is related to us not populating internal action PK / ID field (or similar) when saving the workflow for the first time and so this data doesn't get sent along with subsequent save requests so it fails with a non-unique error (instead of updating an existing action, it tries to create a new one).

I believe this is a blocker because it means I can't work on new workflows "incrementally".

[LindsayHill]

Probably same as #274