volume-map and volume-ephemeral-storage should error out if one of the two wasn't specified

While trying to use a nova volume as the storage of the charm, I ended up with the postgresql unit in a error state.
It happened because I first set the volume-map key to something like {postgresql/0: vol-000001} and only then I set volume-ephemeral-storage to False.

Steps to reproduce:

1) Deploy the PostgreSQL charm with: juju deploy postgresql

2) Attach the nova volume to the PostgreSQL unit instance id under /dev/vdc

3) Run: juju set postgresql volume-map="{postgresql/0: vol-00001}"

4) Run: juju set postgresql volume-ephemeral-storage=False

5) Run: juju status

Verify that the PostgreSQL unit is in error state as the following:

agent-state-info: 'hook failed: "config-changed"'

6) Run: juju resolved --retry postgresql/0

The unit will remain in the same error state.

It turns out that if both key/values are set at once, it works. So by deploying in the following order works as expected.

1) Deploy the PostgreSQL charm with: juju deploy postgresql

2) Attach the nova volume to the PostgreSQL unit instance id under /dev/vdc

3) Run: juju set postgresql volume-map="{postgresql/0: vol-00001}" volume-ephemeral-storage=False

'juju status' should report no errors this time.