check for correct version values when parsing easystack file

[boegel]

This should help a lot to avoid problems like the ones reported in #3687, since you'll get a clear error message like this:

$ cat test.yml
software:
  binutils:
    toolchains:
      SYSTEM:
        versions:
          2.32:

$ eb --easystack test.yml --experimental
== Temporary log file in case of crash /tmp/eb-jy8oppza/easybuild-gjc50tnl.log
ERROR: Value 2.32 (of type <class 'float'>) obtained for binutils (with system toolchain) does not represent a valid version!
Make sure to wrap the value in single quotes (like '2.32') to avoid that it is interpreted by the YAML parser as a non-string value.

and

$ cat test.yml
software:
  Autotools:
    toolchains:
      SYSTEM:
        versions:
          20150215:

$ eb --easystack test.yml --experimental
== Temporary log file in case of crash /tmp/eb-5qedfcz2/easybuild-tzj9ie4u.log
ERROR: Value 20150215 (of type <class 'int'>) obtained for Autotools (with system toolchain) does not represent a valid version!
Make sure to wrap the value in single quotes (like '20150215') to avoid that it is interpreted by the YAML parser as a non-string value.

[branfosj]

Going in, thanks @boegel!