COPR's `gpgkey` field should honor the `requires sign` option for community products

After #293 is implemented, we need to ensure that `gpgkey` field in resulting COPR repo file follows the option "requires sign", this is:
* When sign is required, the COPR repo file should include the `gpgkey` field indicating where the gpg key is located and `gpgcheck` should be set to `1`
* When sign is not required, the `gpgkey` field should be absent and `gpgcheck` should be set to `0`