Ability to override package exclusion list on a per repository basis and toggle gpg check

Added two new attributes to the yum provider (exclude_list and
gpgcheck_enabled)

:exclude_list gives the ability to override the package exclusion list
on a per repository basis. A use case I have is to add the exclusions
to a base role so that it transcends my entire infrastructure. e.g

"default_attributes": {
"yum": {
"epel": {
"exclude_list": [
"rabbitmq-server*",
"v8",
"node",
"jdk"
]
}
}
}

:gpgcheck_enabled - Gives the ability to run a repository without GPG
keys.

Added example to the epel attributes