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

This commit does not nothing when using older slapproxy.

/reviewed-on nexedi/slapos.toolbox!62

/reviewed-on nexedi/slapos.toolbox!63

/reviewed-on nexedi/slapos.toolbox!51

/reviewed-on !59

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

/reviewed-on nexedi/slapos.toolbox!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 !55