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

[javihernandez]

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