Allow IPSec encryption only for GRE tunnel

[jianjuns]

IPSec encryption works for only GRE tunnel, so changes Agent config
valiation to allow EnableIPSecTunnel only when the tunnel type is
set to GRE.
Also improve the logging config of strongSwan charon daemon.

[antrea-bot]

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[jianjuns]

Although IPSec should be supported for all tunnel types according to the OVS documentation (http://docs.openvswitch.org/en/latest/howto/ipsec/), it never works for other types except GRE in all my tests. So, before we figure out the problem, I would disallow IPSec enabled for other tunnel types.

[jianjuns]

/test-all

[tnqn]

LGTM

[jianjuns]

/test-all

[jianjuns]

Corrected a typo in the commit message.

[antoninbas]

LGTM