ImportError: No module named lxml.etree

Hi
I was trying to setup CBT on a fresh system and encountered the following problem

```
[root@ceph-node1 cbt]# python cbt.py
Traceback (most recent call last):
File "cbt.py", line 9, in
import benchmarkfactory
File "/root/cbt/benchmarkfactory.py", line 10, in
from benchmark.cosbench import Cosbench
File "/root/cbt/benchmark/cosbench.py", line 8, in
import lxml.etree as ET
ImportError: No module named lxml.etree
[root@ceph-node1 cbt]#
```

The fix was simple `yum -y install python-lxml`, just for documentation purpose i am creating this issue. I will submit a PR for updating documentation.

Review the setup path from cbt.py to benchmark/cosbench.py, where the lxml.etree import is shown, and identify the appropriate existing setup documentation. Document that Python lxml is required and that installing python-lxml resolves this import error; done means a fresh setup lists the dependency clearly.

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