Refactor MySQL long text support

Seperate Json into two classes: LongText and Json. So type LongText
can be reused in both models and migration, for example, reused by
software config model(config).

Change-Id: If6a8f77e656ad17ee465cb4724d5e28962c0efef
Fix incorrect resource types in docs/comment/code

The template guide documentation references OS::Compute::Server in several
places, and there are similar errors in the environment code comments and
environment test code. Also there is a similar issue with template guide
reference to Rackspace::Compute::Server, and a minor typo/spelling issue.

Change-Id: Iafbf405a328bdd8222c277efe5b52d7f8f589635
Closes-Bug: #1270078
Merge "Fix incorrect resource types in docs/comment/code"
Merge "Refactor MySQL long text support"