slapos node software fail with:

```
We have no distributions for lxml that satisfies 'lxml==3.7.3'.
Building lxml version 3.7.3.
Building without Cython.
ERROR: /bin/sh: 1: xslt-config: not found

** make sure the development packages of libxml2 and libxslt are installed **

Using build configuration of libxslt
Compile failed: command 'gcc' failed with exit status 1
*********************************************************************************
Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?
*********************************************************************************
An error occurred when trying to install lxml 3.7.3. Look above this message for any errors that were output by easy_install.
While:
  Installing.
  Getting section instance-profile.
  Initializing section instance-profile.
  Getting section proftpd-output.
  Initializing section proftpd-output.
  Getting option proftpd-output:command.
  Getting option proftpd-output:proftpd.
  Getting section proftpd.
  Initializing section proftpd.
  Getting option proftpd:environment.
  Getting section proftpd-userinfo.
  Initializing section proftpd-userinfo.
  Installing recipe slapos.cookbook.
Error: Couldn't install: lxml 3.7.3
```