Rename internal types in st2client from Trigger to  TriggerType

- This way the server and client types use identical names. The only place
  we distinguish is in the display strings. Long term should help avoid
  some confusion.
Add Trigger model to client and alias it as TriggerSpecification.
Add Trigger manager
getspecs subcommand on st2 triggers.

- Returns the Trigger specification that make up a concrete Trigger.
Undo unintended change.
TriggerSpecification to Trigger Specification
Merge pull request #905 from StackStorm/trigger-visbility-improvements

[RFR] Trigger visbility improvements