restapi: allow client to define the HTTP session TTL #921013

Session TTL can be explicitly set by user now via "Session-TTL:xxx"
HTTP header,

Session-TTL is the time between client requests before the servlet
container will invalidate this session. An interval value of zero
or less indicates that the session should never timeout.

(default TTL is 180 min)

https://bugzilla.redhat.com/show_bug.cgi?id=921013

Change-Id: I753a379c60b2d7f88867cd0123ee69321a331f0f
Signed-off-by: Michael Pasternak <mpastern@redhat.com>