The generation of the link and the verification were inconsistent, this commit
   normalize both.

   PS.: Software Links are deprecated, it should be removed in future, as it lost
   the goals

  It is costly and inneficient to use.

With slapos.core!125,
this is not the case anymore by default.

This commit does not nothing when using older slapproxy.

/reviewed-on !62

/reviewed-on !63

/reviewed-on !51

/reviewed-on nexedi/slapos.toolbox!59

promise-timeout provides global timeout for the promise, so use it as
base for calculating default timeout.

/reviewed-on !58

Enhancements:

 * implement ignore-code
 * cover http_code
 * cover timeout
 * implement verify
 * correctly cover ca-cert-file
 * use SLAPOS_TEST_IPV4

Improvements:

 * flake8ize test_check_url_available and check_url_available
 * cover more of check_url_available

It will be easier to improve it soon.

Additionally print exact exception message.

/reviewed-on nexedi/slapos.toolbox!56

support broken symlink and don't care about modified files that wouldn't be transferred anyway.

/cc @jm for python review :)
/cc @luke @Nicolas to be sure it's what we want

/reviewed-on nexedi/slapos.toolbox!55