Support for context, architecture and EOLs
Adding three new values to the specification:
* context uniquely identifies a specific module build
among other variants with the same stream, name and version
* arch specifies the content hardware architecture binary
compatibility; note it's not the "basearch"
* eol is a date of when the module goes EOL
This commit includes code changes to cover these additions.
Signed-off-by: Petr Šabata <contyk@redhat.com>