Our python interpreter uses ncclient which depends on lxml egg, but does not
explicitly specify to use that lxml from slapos component.

Until now we were lucky because slapos-cookbook depends on the correct lxml and
we have slapos-cookbook as the second entry in the part list with only
`template` preceding it.

However I needed to use pythonwitheggs inside that template and then got the
build failure, because now pythonwitheggs was built before slapos-cookbook and
tried to use lxml without slapos component:

    INFO Building without Cython.
    INFO Error: Please make sure the libxml2 and libxslt development packages are installed.
    INFO An error occurred when trying to install lxml 4.9.1. Look above this message for any errors that were output by easy_install.
    INFO While:
    INFO   Installing pythonwitheggs.
    INFO   Base installation request: 'websocket-client', 'pynacl', 'bcrypt', 'xmltodict', 'ncclient'
    INFO     Requirement of ncclient==0.6.13: six
    INFO     Requirement of ncclient==0.6.13: lxml>=3.3.0
    INFO     Requirement of ncclient==0.6.13: paramiko>=1.15.0
    INFO     Requirement of ncclient==0.6.13: setuptools>0.6
    INFO     Requirement of bcrypt==3.1.4: six>=1.4.1
    INFO     Requirement of bcrypt==3.1.4: cffi>=1.1
    INFO     Requirement of pynacl==1.3.0: cffi>=1.4.1
    INFO     Requirement of pynacl==1.3.0: six
    INFO   Getting distribution for 'lxml==4.9.1'.
    INFO Error: Couldn't install: lxml 4.9.1

-> Fix it by explicitly specifying in-slapos lxml egg to be used.

/cc @jhuge, @Daetalus
/reviewed-by @lu.xu, @xavier_thompson
/reviewed-on nexedi/slapos!1469