Created mesos::modules namespace for all module related code.

Review: https://reviews.apache.org/r/27020
Checking "kind" before creating a module instance.

Introduced a new kind<T>() function that returns the string
representation of the module "kind". This function is specialized for
each module "kind". Also introduced a templatized
contains<T>(moduleName) that returns true only if a module of name
"moduleName" and kind "T" is available.

Review: https://reviews.apache.org/r/26906