keychain: define SHORT_HOST if not defined

Fixes problems when using the plugin via antibody or antigen.

plugins/keychain/keychain.plugin.zsh

+# Define SHORT_HOST if not defined (%m = host name up to first .)
+SHORT_HOST=${SHORT_HOST:-${(%):-%m}}
+
 function _start_agent() {
 	local agents
 	local -a identities