container/lxc: hard code loop device manager

[davecheney]

lxx.NewContainerManager requires a loop device manager. This parameter
is not documented, and there is no check to ensure that it is not nil.
There is only one caller of this method, inside container/factory which
always passes a valid value. This parameter tf. is only used for testing.

This change hard codes the parameter unconditionally passed in via the
the container factory and adds a new test scoped method to alter the
loopback manager for testing.

(Review request: http://reviews.vapour.ws/r/4146/)

[davecheney]

$$JFDI_$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Does not match ['fixes-1556630']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/6801

[davecheney]

$$JFDI$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju