ITS#7360 accept nss certname in the form of tokenname:certnickname

There are cases where the user may want to force the use of a particular
PKCS11 device to use for a given certificate.  Allow the user to do this
with MozNSS by specifying the cert as "tokenname:certnickname" where
token name is the name of a token/slot in a PKCS11 device and certnickname
is the nickname of a certificate on that device.