Configurer improved version matching (allow RHEL >= 7.4, < 8.0)

[kke]

Makes it possible to define more advanced host configurer version matchers. Currently it's only possible to list each version separately:

Before:

class Rhel7 < Configurer
  register_config 'rhel', '7.4'
  register_config 'rhel', '7.5'
  register_config 'rhel', '7.6'
  register_config 'rhel', '7.7'
end

After:

class Rhel7 < Configurer
  register_config 'rhel', /^7\.[4-9]|\d{2,}/
end

This has been used in this PR to make the rhel7 configurer match RHEL >= 7.4 && < 8.0. (Including something like 7.19-beta2.rc4)

The matcher can be anything that handles triple equals.

[kke]

Unrelated, but I don't see why it should iterate through the same class multiple times when searching for a match.

[peterwvj]

Any progress on this? Is it possible that we could merge this any time soon? @kke @jakolehm Thanks in advance.