Base class for common behaviors of PostgreSQLRole

Moved common behavior among distributions of
PostgreSQLRole to a new base class.

To make possible code reuse by other distributions

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
Added PostgreSQLROle for centos

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
Improved tests of centos PostgreSQLRole

Thanks @diogobaeder for the feedback

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
Finished tests for centos PostgresSQLRole

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
Add test for PostgreSQLRole._execute

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
PEP8 and docstring fixes

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
Removed docstrings from BasePostgresSQLRole

Signed-off-by: Fernando Rocha <fernandogrd@yahoo.com.br>
Merge pull request #87 from fernandogrd/base_classes

PostgresSQLRole for Centos