Change composition of classes in order to resolve containment issues

[lruslan]

1. replace 'include' with 'contain' - this will make all subclasses of the module to be contained by base class
2. make plugins follow general sequence and reload stackdriver services in uniform way
3. stackdriver::config::redhat and stackdriver::config::debian explicitely require $apikey (otherwise stackdriver agent won't start)
4. remove inheritance from base class "inherits stackdriver", as it caused all containment issues. Subclasses should access 'global' level parameters from 'class stackdriver' using full name: $stackdriver::ensure, $stackdriver::svc
5. moved installation and configuration parts to own submodules, just to keep base class cleaner:
   contain stackdriver::install
   contain stackdriver::config
   contain stackdriver::service

[dstockman]

Hey @lruslan, can you fix up this PR and resend? I'll get it pulled in ASAP.