add new variable CONAN_CUSTOM_CONANFILE_NAME_OVERRIDE

[SSE4]

currently, CPT always uses conanfile.py as name of conan file containing ConanFile class.
this works flawlessly for the most of use cases, however for the use case installer + library in the same repo (e.g. bison, flex, vulkan, ICU, etc) it's not enough, obviously.
I propose to add new variable named CONAN_CUSTOM_CONANFILE_NAME_OVERRIDE to override default.
related code is around here:

conan-package-tools/cpt/runner.py

Line 89 in 14c8688

r = self._conan_api.create(".", name=name, version=version,

as you can see, it just always invokes conan create . via conan API, and there is no way to use different file name.
in general, I prefer to leave my build scripts untouched (so they match templates), and just specify variable name in travis matrix, like:

CONAN_CUSTOM_CONANFILE_NAME_OVERRIDE=bison.py
CONAN_CUSTOM_CONANFILE_NAME_OVERRIDE=bison_installer.py

/cc @uilianries

Description of Problem, Request, or Question

Environment Details

• Conan Package Tools Version: 0.23.0
• Operating System: Linux
• Operation System Version: Ubuntu 16.04
• Compiler+version: gcc-5.4
• Docker image: conanio/gcc7
• Conan version: conan 1.2.0
• Python version: python 3.7.0

Steps to reproduce (Include if Applicable)

Build logs (Include if Available)

[uilianries]

I agree, but could we use CONAN_CONANFILE instead of CONAN_CUSTOM_CONANFILE_NAME_OVERRIDE ?

[SSE4]

ok