gPlazma: add public key size information to gPlazma results printer

Recent changes to JVMs have shown that key-size can be an importact
factor when understanding why a login has failed; specifically,
new JVMs now disallow certificates with RSA keys with less than 1024
bits.

This patch adds support for printing the number of bits in the public
key(s).  Unfortunately the interface (PublicKey) provides no mechanism
for discovering this information so we must down-cast to known
classes to discover the information.

Requires-notes: yes
Requires-book: no
Target: master
Request: 2.7
Request: 2.6
Request: 2.2
Merge pull request #321 from paulmillar/fix/2.7/add-pub-key

gPlazma: add public key size information to gPlazma results printer