Move flow states to developers documentation

- Move Flow States documentation from wiki to development docs
- Add Retry States section

Change-Id: Ie60dbbae9290ed3a49e6e58037a1a977a83850d8
Add Retry to developers documentation

Change-Id: I1e97af1f2e5d34b3e403c8c06d56f5d6157a7402
Add more tests for resumption with retry

Cover cases where crash happens at different point of atom
failure processing.

Change-Id: I3ba1cacbcec2a052e91f508debf3fef9b04c3d42
Flattening improvements

- remove extraneous locking from flattener;
- don't add 'flatten': True to edge data: it means nothing currently
  as all the edges get it;
- improve unit tests for flattening:
  - check retry was correctly set with assertIs instead of assertEqual;
  - check exact source, destination and attributes of resulting
    graph edges;
  - cover more error paths;
- minor code cleanups.

Change-Id: Iab23e245cdc834e2cdcb4b7011e13be59aca754a
Add __repr__ method to Atom

Having atom name in its representation helps a lot
when debugging.

Change-Id: I8cb7054b95a8810a7fe3893ba579e9f9e9facd8e
docs: Links to methods on arguments and results page

Make all references to execute() and revert() methods of BaseTask
and Retry links to the method documentation.

Change-Id: I4396daadcf1e1c0e9a9def51d79975f1c0030787
Merge "Add __repr__ method to Atom"
Merge "Flattening improvements"
Merge "Move flow states to developers documentation"
Merge "Add Retry to developers documentation"
Merge "docs: Links to methods on arguments and results page"
Merge "Add more tests for resumption with retry"