Add optional keyfile argument to rake tasks

This addition walks over any extra arguments provided to the
rake task. If the file is a key file, it is used to set the
BEAKER_keyfile ENVIRONMENT variable for beaker-rspec and/or
the `--keyfile` command line argument for beaker as needed.

Example:

BEAKER_setfile=../vcenterhost.cfg rake beaker:rspec:test[foo,pe,'/home/myuser/.ssh/id_rsa-secret']
Update tests for ubuntu 14.04

In ubuntu 14.04 git 1.9.1 is the default version, and 1.9+ fatally fail
if the user.email and user.name are not set (previously it would just
warn). This commit sets those up so the tests will actually run.
Merge pull request #151 from hunner/fix_trusty

Update tests for ubuntu 14.04
Fix logic to validate keyfile
Merge pull request #150 from johnduarte/rake_keyfile

Add optional keyfile argument to rake tasks