Fix LVM unit tests.

[BobBall]

The UT depend on a specific path for lvcreate binaries, however
the path actually used is distro dependent. On Debian distros,
lvcreate is in /sbin. SM correctly auto detects the location,
but the UT were expecting it in /usr/sbin.

Fix the UT so they use the detected path for the executables.

[coveralls]

Coverage increased (+0.01%) to 28.746% when pulling bffb97f on BobBall:lvm-UT-fix into dd3e818 on xapi-project:master.

[germanop]

@MarkSymsCtx this seems reasonable to me but I'd rather have you looking at it

[MarkSymsCtx]

If we need a quick fix then this change is OK.

However, as the unit tests are apparently distro dependent this means that they are not correctly isolated from the execution environment and are accessing the local filesystem which should not be the case. So we can either approve this as a short term fix and raise a new issue to correctly isolate the tests, which may be non-trivial depending on how twisted the logic is, or we reject and only do the test rework to isolate.

[BobBall]

The real issue is that https://github.com/xapi-project/sm/blob/master/drivers/lvutil.py#L40 attempts to detect the location of lv executables on file import.
This means the UT have no real opportunity to provide a stable environment.
Using the detected path seems to be the least-worse change as it doesn't change the functionality.

After approving this change a refactor to fix the lvutil code would at least have UT validating the refactor.