Support for new secret type and fix the multi-tenant build support.

Implementing a new secret type to use the host environment variables #53

Documentation updated how to use secrets.
Added an example how to use host environment variables as secrets: https://github.com/cloud66/habitus/tree/master/examples/security_env

fix multitenant builds #62
fix the problem with dovetailing builds.
Add an example how to use dovetailing.
https://github.com/cloud66/habitus/tree/master/examples/uid_nested

some beautification of logging