add `fab <env> manage` for running mgmt cmds
add `fab <env> manage` for running mgmt cmds

e.g.
```bash
fab production manage --set cmd='prune_couch_views'
```

dimagi/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,qedsoftware/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq