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
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
Merge pull request #9336 from dpkirchner/devel

Added _meta hostvars key to gce.py plugin
Add host to polling message

The message for when an async task has finished has the hostname in it. This adds it to the polling message as well:

```
<job 390794962174.18311> finished on 107.6.24.140
<job 390794962174.31779> finished on 69.90.50.171
<job 390794962174.12328> polling on 69.90.50.172, 6840s remaining
```