conf.d support

Allow Swift daemons and servers to optionally accept a directory as the
configuration parameter.  Directory based configuration leverages
ConfigParser's native multi-file support.  Files ending in '.conf' in the
given directory are parsed in lexicographical order.  Filenames starting with
'.' are ignored.  A mixture of file and directory configuration paths is not
supported - if the configuration path is a file behavior is unchanged.

 * update swift-init to search for conf.d paths when building servers
    (e.g. /etc/swift/proxy-server.conf.d/)
 * new script swift-config can be used to inspect the cumulative configuration
 * pull a little bit of code out of run_wsgi and test separately
 * fix example config bug for the proxy servers client_disconnect option
 * added section on directory based configuration to deployment guide

DocImpact

Implements: blueprint confd
Change-Id: I89b0f48e538117f28590cf6698401f74ef58003b
Merge "conf.d support"