Variable length array of std::strings (not legal in C++) changed to std::vector<std::string>

Signed-off-by: Daniel J. Hofmann <daniel@trvx.org>
Merge pull request #1768 from daniel-j-h/code_quality

Variable length array of std::strings (not legal in C++) changed to std::vector<std::string>

Reviewed-by: Sage Weil <sage@inktank.com>