Made load_balencers in ec2_lc non-required as not all ASGs require an ELB

Add GCE guide and retool a bit to show the add_host interactions, improvements/upgrades are welcome.

Had to shoot the recently merged nova_group module in the head temporarily as it contained a dict comprehension, which means it can't work on all the platforms
and was also breaking docs builds on CentOS.  Will engage with list about that shortly.

Tests several ways to specify the repository.  For every repo added, the test
asserts that:

 * the apt-cache was updated as expected (depends on `update_cache` parameter)
 * the PPA key was installed (depends on `repo` format)

[apt_repository] Correct check_mode conditional logic

The desired behavior is to _not_ add the ppa signing key when check_mode is
enabled.  This fix corrects the conditional logic to comply with the stated
behavior.

Properly catch import errors in apt

When one accidentally tries to run this module as a user, he gets the error message that python-apt must be installed, no matter what. Because importing apt will trigger an exception as a regular user. Explicitly catching the ImportError will let the exception bubble. The exception clearly says Permission denied somewhere, and the user has a better idea, what he must fix.

apt_key: Fix traceback when key_id format is invalid

Fix documentation for accept_hostkey in GIT module

Documentation updates

This allows the use of an args list with leading environment variables,
which otherwise would fail due to the way Popen works.

This reverts commit 4273cb2d.

This allows the use of an args list with leading environment variables,
which otherwise would fail due to the way Popen works.

To support parallel cloud test execution, create and provide a random string to
cloud integration tests.  The variable 'resource_prefix' can be used in cloud
roles and during resource cleanup to safely create/destroy cloud-based
resources.

Additional changes include:

 * The roles test_ec2_key and test_ec2_group were updated to use to
 {{resource_prefix}}.

 * Additionally, the Makefile was updated to set resource_prefix to a random
 string.  The Makefile will also use 'resource_prefix' during cloud_cleanup.

 * All test_ec2* roles were updated to add 'setup_ec2' as a role dependency.

Signed-off-by: Philippe Pepiot <phil@philpep.org>

This reverts commit 3cd7d47b.

Modules make run command safer

Tests clean.

Fixes various typos, fetch_url now working, changed command composure for wait_for_service