extract configuring centos for libyaml + empty redhat wrapper
move checking if centos libyaml-devel package is available to common function
extract error message for missing centos libyaml-devel + improved handling
initial take at rhn-channel as source of libyaml-devel
do not try manually compile yaml on redhat
do not error out when optional redhat repositories are enabled
install libffi-devel always on centos likes
move checking centos libyaml repositories to before action
remove the check for requirement of libyaml-devel when running reporsitories installation on centos
Revert "remove the check for requirement of libyaml-devel when running reporsitories installation on centos"

This reverts commit 00f5cb52c801112224628f5ce33c9f8f43058c04.
add requirements step before_update and move the centos repositories initialization to it
reworked checking if extra repositories are needed on centos-likes
move up the requirements_centos_before_update just after functions it calls
improved check for missing packages and extracted it to new function
improved naming of centos functions to install repositories
do not log the rhn-channel enabling to avoid missing Username problem
merge #3194: use rhn-channel for libyml-devel