Better support Namespaces with drush

Problem/Motivation

Opening this thread to post commits relevant to improving the namespace support for drush.

Proposed resolution

1. Remove the kludged getFullConfigCollection() added in #2455121: Add "drush fc" and "drush fa" commands
2. Fix getExistingPackages() to filter on namespace as mentioned in #25 of #2401583: Recognize existing modules in the curent feature set
3. Add the --all and --name arguments to drush as mentioned in #2453111: Feature "sets" and namespacing. Make this a global argument for all features-releated drush commands.

API changes

1. FeaturesManager::applyNamespace($namespace = NULL)
   Applies the specified namespace and runs all the assignment methods. If $namespace is null, use the existing profile machine_name. If $namespace is an empty string, clears the namespace and causes all config to be detected.
2. Add a $namespace argument to FeaturesManager::getExistingPackages($enabled = FALSE, $namespace = NULL)