Add recursive variant of Config.getNames() methods

Add recursive variant of Config.getNames() methods

These methods allow to find all configuration entry names for a given
section or section/subsection searching recursively through all base
configurations of the given configuration.

These methods are needed to calculate the names for the effective
configuration of a git repository which combines the configuration entry
names found in the repository, global and system configuration files

Bug: 396659
Change-Id: Ie3731b5e877f8686aadad3f1a46b2e583ad3b7c6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>