vyatta-op-vpn: prevent invalid rsa key file from being generated

If the command "generate vpn rsa-key" is aborted during key generation
it leaves behind a temporary file.  If the command is then executed
again, this temporary file is appended to rather than being replaced,
resulting in a key file with an extra : RSA { line at the beginning.

This patch checks if this temporary file exists, deleting it if it
does.

Bug #262 http://bugzilla.vyos.net/show_bug.cgi?id=262
vyatta-op-vpn: display the config path location for the rsa key file

The command "show vpn ike rsa-keys" currently displays the full system
file path rather than using the shorter config path.  This sets it to
display the config path instead.

Bug #278 http://bugzilla.vyos.net/show_bug.cgi?id=278