Add system property role.create-application.enabled to control whether anyone can create application or not.

Add system property role.manage-app-master.enabled to control whether any app master can add/delete other app master

Fix SSRF, resolve #2103

feature: diff configurations among clusters. issue #1966

fix bug (when use reverse proxy with TLS, "HttpServletResponse.sendRedirect" will redirect to http rather than https)

1. fix flyway migration Chinese garbled issue
2. limit hibernate query plan cache
3. fix import/export api
4. wording update for yaml/yml namespace support
5. update ldap sample configurations

1. support yaml config file transform to properties
2. support yaml config file injection to Spring

allow config value to be empty. #1882

adjust cache control so that entry htmls are not cached, as we need them to trigger 302 in case user is not logged in

apollo-portal to support displaying configs only to team members, users could control this behavior by configuring configView.memberOnly.envs in ApolloPortalDB.ServerConfig.

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

1. Remove apollo-env.properties from apollo-core, so that it's possible
to deploy apollo-client to maven central repository.
2. Add MetaServerProvider spi so that users could customize how they
find apollo meta servers.
3. Refactor env related utils so that Env.UNKNOWN is returned for
invalid env string instead of null.