Use self. for clarity in class method definition

class << self is confusing for many people
Refactor and improve Host and Role Filtering

Update CHANGELOG and README
Replace HostFilter and RoleFilter with Filter in Configuration
Create list of filters from command arguments and ENV vars
Intercept the existing on() method to apply external filters
Update roles_for() and fetch_primary() to use internal filters
Improve Filter code

Split only valid lists of host names to avoid splitting a regex
containing a comma.
Define // syntax for regex in role filters
Treat all host/role filtering options the same

Command line —host and —role options
Environment variables ROLES and HOSTS
Filter variable hash keys :role and :host
Override netssh_options and use super

The existing method of using aliases is confusing
and unnecessary
Fix bug with filtering single server

Manifested by on primary() construct
Merge pull request #1140 from townsen/on_filters

Host and Role filtering improvements