Enabling separated sample config file generation

As a common approach most projects used now and Oslo preferred, this
change enabled sample configuration file generation mechanism for
each Glance services.

This change, as an enhancement, allows generating separated sample
configuration files for each Glance major services, e.g.:

    etc/glance-api.conf.sample
    etc/glance-cache.conf.sample
    etc/glance-manage.conf.sample
    etc/glance-registry.conf.sample
    etc/glance-scrubber.conf.sample

It is different than I94d486d6686815c45705a7a9b00fb26062e1eb63
which only supports generating an unified sample configuration
file to including all Glance available options.

This mechanism  not only can help auditing by packager, milestone
maintainer or developer as a function (testenv) of tox, but also
those separated configuration files could make deployment be easy.
And it helps keeping sample configuration files be update with
code change.

The change added "genconfigs" function as a tox testenv (-egenconfigs),
and it dependes on oslo-config-generator function of oslo.config.

The change doesn't introduce those sample files Glance repo, so
next step is to investigate if we can generate them in gate
automatically when a change was merged.

Related-Change-Id: I15686708fc9460948a58cfea3d18dae40ba1fda9
Related-Change-Id: Iae31856d5886ee78786972d80c7c103c3460a2b3
Related-Change-Id: I76043b08e2872867e5af2a5ac902e4d092fda5c8
Closes-Bug: #1300546
Closes-Bug: #1361963

Change-Id: Ibe03a3fe80b96ca32acb1a6bea7e38e6075951bb
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Enabling separated sample config file generation"