Fix EZP-23572: create user and assign role for "I am logged as :role"

JIRA: https://jira.ez.no/browse/EZP-23572

For the `"@Given I am logged (in) as a(n) :role"` sentence, the `'admin'` user credentials were being used.

This PR modifies/fixes this by creating a new temporary user, and assigning it the needed role.
Requires #16 (User object management/sentences)

After the scenario, user is automatically removed.