environs: EnvironProvider.SecretAttrs() strings

environs: EnvironProvider.SecretAttrs() strings

Changed EnvironProvider.SecretAttrs() method to
return map[string]string, instead of map[string]interface{},
as this is a prerequisite step to pluging in the security
issue we have with the provisioner API.

Also, because all providers currently use string values
as secret attributes, this causes no further stir.

https://code.launchpad.net/~dimitern/juju-core/144-environs-secretattrs-strings/+merge/187566

Requires: https://code.launchpad.net/~dimitern/juju-core/144-worker-provisioner-uses-api/+merge/187191

(do not edit description out of merge proposal)

[dimitern, 2013-09-25 17:08:53]

Please take a look.

[dimitern, 2013-09-25 17:50:01]

Please take a look.

[fwereade, 2013-09-25 17:56:53]

Excellent, LGTM.