Add use task subcommand

Added a new subcommand in use command: 'rally use task <task_id>'
which sets a RALLY_TASK global var in ~/.rally/use_command
Hence the user does not need to specify a task UUID in commands
requiring this parameter.

Partially implements blueprint: use-task-subcommand

Change-Id: I51fffa017c56321493d1a5eba866d7163b9fc813
Merge "Add use task subcommand"