Host management Azure driver

Fix formatting, change to markdown block for long

TL;DR: check for IsExist(err) after a failed MkdirAll() is both
redundant and wrong -- so two reasons to remove it.

Quoting MkdirAll documentation:

> MkdirAll creates a directory named path, along with any necessary
> parents, and returns nil, or else returns an error. If path
> is already a directory, MkdirAll does nothing and returns nil.

This means two things:

1. If a directory to be created already exists, no error is returned.

2. If the error returned is IsExist (EEXIST), it means there exists
a non-directory with the same name as MkdirAll need to use for
directory. Example: we want to MkdirAll("a/b"), but file "a"
(or "a/b") already exists, so MkdirAll fails.

The above is a theory, based on quoted documentation and my UNIX
knowledge.

3. In practice, though, current MkdirAll implementation [1] returns
ENOTDIR in most of cases described in #2, with the exception when
there is a race between MkdirAll and someone else creating the
last component of MkdirAll argument as a file. In this very case
MkdirAll() will indeed return EEXIST.

Because of #1, IsExist check after MkdirAll is not needed.

Because of #2 and #3, ignoring IsExist error is just plain wrong,
as directory we require is not created. It's cleaner to report
the error now.

Note this error is all over the tree, I guess due to copy-paste,
or trying to follow the same usage pattern as for Mkdir(),
or some not quite correct examples on the Internet.

[v2: a separate aufs commit is merged into this one]

[1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go

Signed-off-by: Kir Kolyshkin <kir@openvz.org>

Really fixing 2 things:

1. Panic when any error is detected while walking the btrfs graph dir on
removal due to no error check.
2. Nested subvolumes weren't actually being removed due to passing in
the wrong path

On point 2, for a path detected as a nested subvolume, we were calling
`subvolDelete("/path/to/subvol", "subvol")`, where the last part of the
path was duplicated due to a logic error, and as such actually causing
point #1 since `subvolDelete` joins the two arguemtns, and
`/path/to/subvol/subvol` (the joined version) doesn't exist.

Also adds a test for nested subvol delete.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>