Alter escaping in postgresql::config::afterservice

This allows non-standard users (pe-postgres) to change passwords. Also
added a function to do escaping of the password, added system tests
and rspec tests for the function.
reverting path on exec
amending the unless on the exec for quoting also
modified tests
Merge pull request #218 from fiddyspence/set_postgres_postgrespw

Alter escaping in postgresql::config::afterservice