tests/run: Make IAM role changes optional
tests/run: Make IAM role changes optional

I'm not authorized to assume roles in the teamcoreservices account,
and the assume-role call errors out with:

  An error occurred (AccessDenied) when calling the AssumeRole
  operation: Not authorized to perform sts:AssumeRole

That's fine though; I can still launch the cluster with my usual
access.  This commit makes the role assumption optional.  I've made
setting up the AWS_* access variables conditional on successful role
assumption, because setting them based on an empty $RES wouldn't work
;).

Using the && chain with a terminal || keeps the script from dying on
this assume-role failure.  From the 'set -e' docs [1]:

  The shell does not exit if the command that fails is ... part of any
  command executed in a && or || list except the command following the
  final && or ||...

I'm also only setting iamRoleName configs if assume-role succeeded.
We've been setting iamRoleName since the script landed in a2405e4d
(run smoke tests with bash script, 2018-06-18,
coreos/tectonic-installer#3284) and possibly before that since
82daae14 (tests: add etcd role, 2018-03-14,
coreos/tectonic-installer#3074).  I don't see anything in those
commits or PRs to motivate the iamRoleName entries, but I'd guess
they, like the tf-tectonic-installer role, are specific to the Jenkins
setup.  I've tied them together with CONFIGURE_AWS_ROLES based on that
similarity, although in theory you may be able to toggle the
iamRoleName settings independently of assume-role success.

Even though the &&/|| chain sets CONFIGURE_AWS_ROLES=False when
assume-role failes, I'm using ${CONFIGURE_AWS_ROLES:-False} in the
Python script.  That way, future versions of this script that support
libvirt (or other backends) won't need to bother setting
CONFIGURE_AWS_ROLES and will still get valid Python here.  The :-
syntax is specified in [2], and my expansion defaults to False if
CONFIGURE_AWS_ROLES is unset or empty.

[1]: https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02

derekhiggins/installer,derekhiggins/installer