Improve OS::Trove::Instance resource

Improvements include:
- better handling of possible Trove instance states;
- usage of proper troveclient exception;
- better error messages;
- setting length constraint on user databases property;
- NAME property is no longer required as it can be autogenerated;
- name autogeneration fixed to prevent the like of bug #1324499;
- simplification in tests by removing environment in test template;
- some typos fixed.

Change-Id: I27c569a16a316448001d857e839c9aabc28ca611
Merge "Improve OS::Trove::Instance resource"