This commit adds a `--syntax-only` flag for `promtool check config`.
When passing in this flag, promtool will omit various file existence
checks that would cause the check to fail (e.g. the check would not
fail if `rule_files` files don't exist at their respective paths).
This functionality will allow CI systems to check the syntax of
configs without worrying about referenced files.
Fixes: #5222
Signed-off-by: zzehring <zack.zehring@grafana.com>
