Allow specification of default database name

E.g. pe-postgresql does NOT use postgres as the default database name.
It uses pe-postgres. So if there is no way to specify a default database
name, the postgesql::validate_db_connection resource in
postgresql::server::service will ALWAYS fail. This commit exposes the
parameter in order to avoid that situation.
Add globals/params layering for default_database

This makes the variable consistent with the manner in which most/all of
the rest of the postgresql module currently works.

Commit also adds the new param to the README file.
Use default_database in validate_db_connection

This commit modifies postgresql::validate_db_connection to use the
default_database parameter from postgresql::params rather than
hard-coding a local default value of "postgres".
Merge pull request #298 from reidmv/allow_specification_of_default_db_name

Allow specification of default database name