Update how dashboard's secret key is handled

Currently, you can override the 'secret_key_path' attribute but this
does not get reflected in the dashboard configuration.  This change
does the following:

- updates local_settings.py to use the specified secret key
- adds a default attribute for 'secret_key_content' which when set
overrides the contents of the secret key (useful if you have multiple
web servers running dashboard and they all need the same secret key)

Change-Id: Ia82947280a64f6b1ec693ee273ac386a71be57ad
Closes-Bug: #1279863
Merge "Update how dashboard's secret key is handled"