Update playbooks_roles.rst

Fixes https://github.com/ansible/ansible/issues/7241
Removing hard coded link and replacing with reference
Added _meta hostvars key to gce.py plugin per the discussion in issue #9291.
Add --pretty option for gce inventory plugin
Add integration test for the shell module

* it ensures that it's possible to define options in the multiline block
Typo: greatful -> grateful
Moved v2 tests for plugins to new location
Adding block code and tests
Update block.py

Add note on block properties
Detect kvm guests through sys_vendor.

The two detection methods currently implemented do not cover all cases.

For example qemu guests invoked like this:
qemu -machine pc-i440fx-1.4,accel=kvm -cpu SandyBridge

return this information:
product_name: Standard PC (i440FX + PIIX, 1996)
cpuinfo: model name      : Intel Xeon E312xx (Sandy Bridge)
Detect docker as virtualization_type

New docker versions use "docker" in /proc/1/cgroup, which disturbs the
virtualization_type detection. So, grep for docker, in addition of
"lxc".
Merge pull request #9338 from szkkentaro/gce_inventory/add_pretty_option

Add --pretty option for gce inventory plugin
Merge pull request #9346 from MiLk/test/shell-multiline

Add integration test for the shell module
Merge pull request #9353 from ivuk/fix-typo

Typo: greatful -> grateful
Change "usuable" to "usable".
Docstring for v2 inventory.subset
Merge pull request #9381 from debfx/facts_virt_kvm

Detect kvm guests through sys_vendor.
Merge pull request #9382 from gquintard/WIP/newdockerdetection

Detect docker as virtualization_type
Overhauls to v2 code

* using inspect module instead of iteritems(self.__class__.__dict__, due
  to the fact that the later does not include attributes from parent
  classes
* added tags/when attributes to Base() class for use by all subclasses
* removed value/callable code from Attribute, as they are not used
* started moving some limited code from utils to new places in v2 tree
  (vault, yaml-parsing related defs)
* re-added ability of Block.load() to create implicit blocks from tasks
* started overhaul of Role class and role-related code
Catch template syntax errors specifically when templating strings

Fixes #9333