Rename instances->task to generalize container jobs

  * This would allow to connect to staging tasks stdout/stderr sockets

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Read staging tasks stdout/stderr output along with instances

[#63400876]

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Send staging messages with STG source name

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>