Nodepool dependens indirectly on lxml

Nodepool now depends indirectly on lxml via keystoneclient.  To pip
install lxml you need libxml and libxslt development packages on the
system to provide headers so that the library bindings can be
compiled, so it's easier to just use the python-lxml system packages
instead.

Co-Authored-By: Jeremy Stanley <fungi@yuggoth.org>
Change-Id: Id5c2070250cd9474c83edaa6ed1119a5325ea913
Merge "Nodepool dependens indirectly on lxml"