Add resource to extract assembly logs

[Devdatta Kulkarni]

For a given assembly, we want to extract the logs associated with it. Such logs are generated in different stages of an assembly:
- running CI
- creating the DU
- deploying the Heat stack

and so on.

There is already a spec approved for this work:
https://github.com/stackforge/solum-specs/blob/master/specs/juno/stage-logs.rst

In there, the following URI scheme has been proposed for extracting logs:

SOLUM/v2/assemblies/<assembly>/logs/<stage>/<iso8601>.log

This wishlist bug is to implement this scheme.

Note: Solum has a notion of 'sensor', which was initially thought as something we would use for referring to any 'sensing' associated with an assembly. There was some discussion around how logs are one such kind of sensors.
For now, we are not going to pursue the option of using sensors for this purpose. There are several reasons for this:
1) The approved spec does not refer to sensors
2) The notion of sensors comes from CAMP spec. Solum community is still debating (as of September 10, 2014) the how to best integrate, if at all, Solum and CAMP.

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/122512

[OpenStack Infra (hudson-openstack)]

Change abandoned by Murali Allada (<email address hidden>) on branch: master
Review: https://review.openstack.org/122512
Reason: code already checked in.