Remove lxml as a forced depend

keystoneclient only needs lxml for saml support, which is an
optional contrib piece. Since keystoneclient is massiely used, and
as lxml is a big beefy depend that requires C compilers, move
lxml to test-requriements.

Change-Id: I01b6b702fb8f414884cb1da306d5b95406e86513
Merge "Remove lxml as a forced depend"