Introduce Docker managed plugin for Ceph RBD

[codenrhoden]

Plugin image requires that we have the Ceph CLI/tools in the container,
so this plugin is built off of CentOS 7 instead of Alpine. As such, the
plugin has customized versions of rexray.sh and Dockerfile instead of
the common ones.

Closes #818.

[codenrhoden]

This PR requires thecodeteam/libstorage#576.

I've tested on CentOS7 and Ubuntu 16.04. Also tested to make sure that this image, which is based on Ceph kraken (latest release), works with a host and cluster install of Ceph Jewel (latest stable release). That is likely to be a common scenario, if not the majority of installs.

[codecov-io]

Codecov Report

Merging #898 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #898   +/-   ##
======================================
  Coverage    8.79%   8.79%           
======================================
  Files          25      25           
  Lines        2980    2980           
======================================
  Hits          262     262           
  Misses       2682    2682           
  Partials       36      36

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[akutz]

It looks good to me code-wise, but we should get @cduchesne to test it. After that I say merge away.