Ipsec: Always generate ikey

Some tests use a crypto algorithm with no integrity algorithm.
Generate empty binary strings as fake integrity keys
to keep return values of low level methods consistent.

+ Add return_keys argument to avoid returning long lists.
+ Improve various docstrings.

Change-Id: Idae1877bdde32d194ce4e3bb3053c8dba39d377a
Signed-off-by: Vratko Polak <vrpolak@cisco.com>