GPG: Add support for setting the keyrings to be used for signing artifacts

[ddcprg]

We've had some issues configuring Jenkins and the RPM plugin to work together with the GPG plugin.

One of the issues we had is the fact that the configuration for both plugins differs significantly which makes it difficult for both plugins to coexists in the same project. Basically we have on non-default keyring with our public key and another non-default keyring with the private key. The GPG plugin accepts these 2 files to be configured with the properties defaultKeyring=false, publicKeyring and secretKeyring which would be equivalent to the --no-default-keyring and --keyring options in the CLI - Refer to this page for details

http://maven.apache.org/components/plugins/maven-gpg-plugin/sign-mojo.html

These properties provide powerful customization of the signing process. On other hand, the RPM plugin only supports keypath which is the directory where the plugin will look for both pubring.gpg and secring.gpg not given space for customization of the keyring filenames.

http://www.mojohaus.org/rpm-maven-plugin/rpm-mojo.html

I would be great if both plugin could be customized with at least the 3 options to provide more flexibility and similar configuration for both plugins.