vswitch: Use "ipsec_gre" vport instead of "gre" with "other_config"

vswitch: Use "ipsec_gre" vport instead of "gre" with "other_config"

Previously, a GRE-over-IPsec tunnel was created as an interface with a
"type" of "gre" and the "other_config" column with "ipsec_cert" or
"ipsec_psk" set.  This could lead to a potential security problem if a user
intended to create a GRE-over-IPsec tunnel, but misconfigured the
"ipsec_*" config and created an unencrypted GRE tunnel.

This commit defines an "ipsec_gre" tunnel type, which should prevent
users from inadvertently establishing insecure tunnels.