Fix MismatchError for LB scenario test

It has been observed that at a given iteration only one of the two HTTP
services responds to requests made to the VIP. This is somewhat rare and
it may be caused by the fact that the number of requests being sent is
not high enough for the ROUNDROBIN algorithm to equally distribute requests
across all the members in the pool.

Increasing the number of retries makes the test runtime only slightly
longer, but it should give us more confidence that, if we keep observing
the same type of failure, this can be ruled out.

At the same time, this patch also relaxes the check so that it just verifies
that member pools are load balanced at least once. Verifying that the number
of requests are spread evenly across is always going to be prone to transient
condition and it is beyond the scope of a scenario test that validates whether
Neutron LBaaS is wiring things correctly.

Closes-bug: #1358857

Change-Id: Ibc0aeb092a4643eb151cef8d7d942d9e3d1ed71c
Use shared values for test_security_group_rules

In test_security_group_rules, most created security group rules are
the same. This patch adds shared values and makes these tests use
them for cleanup.

Change-Id: Ie86c33217fc6b065844e31f9b6e37be93322f2cb
Don't parse config file if it doesn't exist

This commit adds a check before we parse the config file to see if the
config file actually exists. If it doesn't we shouldn't try to parse
it. There is still an issue with one scenario test that uses
testscenarios but it doesn't result in an error code being raised by a
test-list so that can be tracked down separately.

Change-Id: Iac31f2edcfbccf8c06d73c6ca01e30c8319c0485
Closes-Bug: #1369118
Merge "Use shared values for test_security_group_rules"
Merge "Fix MismatchError for LB scenario test"
Merge "Don't parse config file if it doesn't exist"
Merge branch 'master' of https://github.com/openstack/tempest