Revert "grunt: allows one to set branding through BRANDING env variable"

This reverts commit 8b2dc78f96723ded77b0e825c3ce6213efee3182. This is
useless as there is no way to pass environment variables during the
build.
grunt: use a global variable for default branding

This is easier to modify this way.
doc: say a word about branding with PaaS
doc: say where to find the resulting app for PaaS