For consistency rename "ex_describe_keypairs" method to "ex_describe_keypair".

Also modify "ex_describe_keypair" method to return key fingerprint in the return
value.

Part of LIBCLOUD-326.


git-svn-id: https://svn.apache.org/repos/asf/libcloud/trunk@1484194 13f79535-47bb-0310-9956-ffa450edef68