provider/manual: don't change config in Open/SetConfig

The manual provider's Open and SetConfig methods were
both using the result of "validate", which replaces
missing manual-specific attributes with their defaults.
Open and SetConfig should not alter the input config.

Fixes https://bugs.launchpad.net/juju-core/+bug/1347715
Merge pull request #483 from axw/lp1347715-provider-manual-open

provider/manual: don't change config in Open/SetConfig

The manual provider's Open and SetConfig methods were
both using the result of "validate", which replaces
missing manual-specific attributes with their defaults.
Open and SetConfig should not alter the input config.

Fixes https://bugs.launchpad.net/juju-core/+bug/1347715