Let finding install classes be more flexible for Fedora (#1155452).

Related: rhbz#1155452

(1) Make it so a valid install class does not have to be named "InstallClass"
which should make inheritance less cumbersome.

(2) Allow putting more than one install class into a single file.

(3) Rename the existing install classes to be <distro>BaseInstallClass.  This
makes it more clear for subclasses where they're coming from.

Port of commit 8c2550d20d7fd8964cd3ec5797ff34949fe0f8fd from master.
A couple updates to installclasses.

Related: rhbz#1155452

Port of commit e8952489ce4c130612d6d06f3bc20155eb5fc102 from master.

* hidden and showHidden should be booleans, not ints in disguise.
* Get rid of default and id.  These are unused.
* Stop marking names for translation and remove mnemonics.  These haven't been
  used since the days of workstation/server/custom selection in the UI.  Does
  anyone else even still remember this?
atomic: set up atomic specific partitioning defaults

Resolves: rhbz#1155452

/boot will be 300M and / LV will be 3G. LVs for docker will be created by special
service using root's VG.