create_key test: add missing tests (#132)

* create_key test: add missing tests

In preparation for future work, add a few tests that are missing from
`create_key`, specifically tests that verify the request.cnf,
permissions.p7s, governance.p7s, and ecdsaparam.

Also rework test module to use module fixtures and multiple smaller test
cases instead of one large one. This makes it easier to add more, and
makes test failures more meaningful.

Signed-off-by: Kyle Fazzari <kyle@canonical.com>

* Remove spurious comment

Signed-off-by: Kyle Fazzari <kyle@canonical.com>