fix the situation when some public app namespace is deleted and recreated in another app

allow admin to create private namespace by default

update apollo logo

merge 1.x to master

support setting apollo system properties via application.properties

support setting apollo.meta, app.id, apollo.cacheDir and apollo.cluster from spring boot configuration files, e.g. application.properties, bootstrap.properties, etc

fix image typo

merge from master

add ConfigChangeListener with optional interested keys

add apollo log

prepare for maven central release

Refactor meta server related logic so that apollo-core is common enough to go maven central repository.