Add Selenium and Firefox to vm to power Behat tests

[arknoll]

Relates to #137

Notes/TODOs from @geerlingguy:

• Update www.drupalvm.com documentation with note about Selenium
• Leave it uninstalled by default (to avoid bloat of Java + FF + etc. for majority of projects).
• Update README.txt with documentation about Selenium
• Add wiki page on using Behat (or other tools?) with Selenium (added)
• Test on Ubuntu 14.04 & 12.04 and CentOS 6 & 7
• Make composer global require'd packages more accessible (see Make globally-required Composer packages accessible to vagrant user #154)

[geerlingguy]

Can this be updated (upstream) to a variable with the name of the role prefixed (e.g. selenium_server_version)? Similar to Drupal modules, it's best to use a prefixed variable name so there are no conflicts between roles (and to clarify what, exactly, the variable is used for in a large config file).

[geerlingguy]

Actually, it looks like the variable is selenium_version upstream... is that correct?

[arknoll]

yeah, it should have been selenium_version. Fixed.

[geerlingguy]

One other question: Would you be willing to (or would you accept a PR from me to) make arknoll.selenium work on RHEL/CentOS? Currently all other configuration in the VM works across platforms, so people can switch to other base boxes if they would like, and I don't want to take away that ability if they want to use selenium.

[arknoll]

@geerlingguy let me give it a shot to get it working on RHEL/CentOS.

[geerlingguy]

@arknoll - Great! let me know when that's done, and I'll un-postpone this PR.

[arknoll]

@geerlingguy Done and tested on CentOS https://galaxy.ansible.com/list#/roles/4340.

[geerlingguy]

Great! I'll merge and do some more testing tonight.

[geerlingguy]

Verified that behat/selenium/firefox/etc. works great across all supported CentOS and Ubuntu versions.