Fix a couple of default config values

The number of options in the config file which must
be set to run Tuskar should be as small as possible.
That's why we should choose reasonable default values:

    - nova_overcloud_config should be an absolute part
      within /etc/tuskar dir

    - username to be passed to Heat client should be
      'admin' rather than 'heat' (it's the admin user
      we are likely to deploy the overcloud on behalf
      of, not heat)

Clarify the meaning of user credentials which are used
to make requests to Heat API.

Change-Id: I8f94b6fbc3ebbb4d9264b9735b32bb232609ef5d
Merge "Fix a couple of default config values"