Using enviromental varables to hide actual password in psql commands

### Motivation

Executing commands by Puppet is crucial, because the most concerns production environments. I wanted to avoid displaing password, even in the form of "encrypted" in the logs and reports. These reports are usually stored for a long time, making it possible vector of attack.

### Design

I decided to design it in such a way as to take advantage of the fact that the environment variables are not displayed by the `Puppet::Util::Execution` - even in debug mode. It displays actual command, in this case: 
```shell
/usr/bin/psql -d base -t -c "SELECT 'something' as \"Custom column\""
```

### Development

1. Edited `postgresql_psql` ruby provider, `postgres::server::role` and `postgresql::server::passwd` class
 1. to use environment variables to store actual passwords
 1. to properly escape `"` sign when passing command to `psql` executable
1. Writen a spec test for `postgresql::server::passwd` and `postgres::server::role`
1. Corrected spec test to `postgresql_psql` ruby provider

### Testing

I've tested those changes by running spec tests on:

* rvm: 1.8.7, env: PUPPET_GEM_VERSION="~> 2.7.0" FACTER_GEM_VERSION="~> 1.6.0"
* rvm: 1.9.3, env: PUPPET_GEM_VERSION="~> 3.5" STRICT_VARIABLES="yes"

I've also tested those changes against a vagrant machine `wavesoftware/trusty64-nocm` using both of settings from `.travis.yaml` with:

```shell
tail -f /var/log/syslog | grep postgres &
puppet apply test.pp --debug
```
a test.pp contents are:

```puppet
include postgresql::server
postgresql::server::db { 'mydatabasename':
  user     => 'mydatabaseuser',
  password => postgresql_password('mydatabaseuser', 'mypassword'),
}
```