ConfigMBean Allows Configuration Drift from MantaClient

[tjcelaya]

While doing some investigation into #311 I noticed we allow editing of quite a few properties through ConfigMBean. As an example of a potential issue, it's currently possible to construct a MantaClient with one manta.key_path value and then change that path to point to a different key without impacting the key actually used for signing requests. A simpler example might be the home value used to list running jobs which is only set on construction of a MantaClient. The expected fix for this issue is at the bare minimum to make the attributes read-only through the MBean but support for tweaking values interactively (like manta.max_connections) and their documentation would be nice.

[dekobon]

Typically, access to MBeans is limited to operators and developers. The only time anyone should be muckering with those values is when doing performance testing. I don't see any problem with changing them to read-only. However, it might be a good way to experiment with a mutable version of a Context.

[cburroughs]

A user might assume that anything implementing SettableConfigContext already is a mutable ConfigContext. Maybe the javadoc should could that? ConfigContextBuilder might have been a name more in line with intended use.