Fix #207 - Define subcommands for describing/running stack tasks

[erran-r7]

Usage

Describe all tasks for a cloud

$ convection describe-tasks
Stack vpc (acmecorp-test-0-vpc) includes the following tasks:
 0. associated private zones with VPC in acmecorp-test-0-vpc
 1. unassociated private zones with VPC in acmecorp-test-0-vpc
Stack sessioncache (acmecorp-test-0-sessioncache) includes the following tasks:
 0. Tasks::TagElastiCache

Describe tasks for a subset of stacks

$ convection describe-tasks --stacks vpc
Stack vpc (acmecorp-test-0-vpc) includes the following tasks:
 0. associated private zones with VPC in acmecorp-test-0-vpc
 1. unassociated private zones with VPC in acmecorp-test-0-vpc

No ability to run all tasks

$ convection run-tasks
No value provided for required options '--stack'

Running a task against a specific stack

$ convection run-tasks --stack vpc
Using the "platform-dev" environment configuration for all operations.
The following tasks are available to execute for the stack vpc (acmecorp-test-0-vpc):
 0. associated private zones with vpc-deadb33f
 1. unassociated private zones with vpc-deadb33f
Which stack task (by number) would you like to execute? [0, 1] 0
task_in_progress  Task associated private zones with vpc-deadb33f in progress for stack vpc.
Running associated private zones with vpc-deadb33f for cloud acmecorp-test-0 in region us-east-1
task_complete  Task associated private zones with vpc-deadb33f successfully completed for stack vpc.

Requirement of picking a valid task

$ convection run-tasks --stack vpc
Using the "platform-dev" environment configuration for all operations.
The following tasks are available to execute for the stack vpc (acmecorp-test-0-vpc):
 0. associated private zones with vpc-deadb33f
 1. unassociated private zones with vpc-deadb33f
Which stack task (by number) would you like to execute? [0, 1] 6
Your response must be one of: [0, 1]. Please try again.
Which stack task (by number) would you like to execute? [0, 1] 0
task_in_progress  Task associated private zones with vpc-deadb33f in progress for stack vpc.
Running associated private zones with vpc-deadb33f for cloud acmecorp-test-0 in region us-east-1
task_complete  Task associated private zones with vpc-deadb33f successfully completed for stack vpc.

[simonirwin-r7]

LGTM