Env from database

[Anilople]

What's the purpose of this PR

In PR #2867, discussion_r363044605

I think we can keep it as add environment in portal when start up, but we support configuring it via System Configuration page, e.g. we add a new configuration item like apollo.portal.meta.servers to store the mapping between env and meta server address.

We will still keep the apollo.portal.envs as it indicates the active environments so that there may be 3 environment mappings in apollo.portal.meta.servers but only 2 active envs in apollo.portal.envs.

If this involves much more effort, then it's okay to leave it when we support add environment in portal when running.

That is this PR doing.

Now user can add their own meta server address in PortalDB.ServerConfig with key apollo.portal.meta.servers value like follow.

{
	"local" : "http://localhost:8080",
	"dev" : "http://dev.com:8080",
	"self-define" : "http://self-define.com:port"
}

When portal start, the value will be read to com.ctrip.framework.apollo.portal.component.config.PortalConfig, then class com.ctrip.framework.apollo.portal.environment.DatabasePortalMetaServerProvider will use those informations.

Which issue(s) this PR fixes:

None.

Brief changelog

The main change under package com.ctrip.framework.apollo.portal.environment.

For add this feature, some constant thing to be dynamic.

1. com.ctrip.framework.apollo.portal.environment.PortalMetaServerProvider from class to an interface. This is for multiple meta server providers ( special for data from the database, i.e com.ctrip.framework.apollo.portal.environment.DatabasePortalMetaServerProvider ).

2. PortalMetaDomainConsts -> PortalMetaDomainService. A constant class to a service, and add multi server providers when the service constructing. Use this service instead old one without change methods

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[coveralls]

Coverage increased (+0.1%) to 54.926% when pulling 472114c on Anilople:env-from-database into 0769c80 on ctripcorp:master.

[nobodyiam]

Thanks! There are some places which need some modifications, but it looks to me overall. So I will merge this pull request and post a new pull request for those minor modifications.