tests: resize root partition when atomic host

tests: resize root partition when atomic host

For a few moment we can see failures in the CI for containerized
scenarios because VMs are running out of space at some point.

The default in the images used is to have only 3Gb for root partition
which doesn't sound like a lot.

Typical error seen:

```
STDERR:

failed to register layer: Error processing tar file(exit status 1): open /usr/share/zoneinfo/Atlantic/Canary: no space left on device
```

Indeed, on the machine we can see:
```
Every 2.0s: df -h                                                                                                                                                                                                                                       Tue May 29 17:21:13 2018
Filesystem                 Size  Used Avail Use% Mounted on
/dev/mapper/atomicos-root  3.0G  3.0G   14M 100% /
```

The idea here is to expand this partition with all the available space
remaining by issuing an `lvresize` followed by an `xfs_growfs`.

```
-bash-4.2# lvresize -l +100%FREE /dev/atomicos/root
  Size of logical volume atomicos/root changed from <2.93 GiB (750 extents) to 9.70 GiB (2484 extents).
  Logical volume atomicos/root successfully resized.
```

```
-bash-4.2# xfs_growfs /
meta-data=/dev/mapper/atomicos-root isize=512    agcount=4, agsize=192000 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=0 spinodes=0
data     =                       bsize=4096   blocks=768000, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal               bsize=4096   blocks=2560, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data blocks changed from 768000 to 2543616
```

```
-bash-4.2# df -h
Filesystem                 Size  Used Avail Use% Mounted on
/dev/mapper/atomicos-root  9.7G  1.4G  8.4G  14% /
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 34f70428521ab30414ce8806c7e2967a7387ff00)