refactor db connections for compute

This commit refactors the db connection params
in openstack::compute from a single parameter
that represents the connection string to
individual parameters for each db_name,
db_user, db_password, and db_host.

This patch is intended for two purposes.

1. now that the compute requires two seperate
db connections (cinder and nova), by breaking them
down to these parameters, they can share their db_host
parameter.

2. it simplifies the anount of information users have to
specify to a password. In most cases, the defaults will
be sufficient for all other data.

Change-Id: I01a5a987a6a5b390b0a132b4f9b3261aae9c1f13