avocado/utils/crypto.py: remove get_random_string function

The exactly same function body is present under avocado/data_factory.py,
under the name generate_random_string.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
avocado/utils/crypto.py: remove compatibility code for older Python versions

Avocado needs Python 2.7.0 and later, so there's no point in maintaining
compatibility code with older Python versions.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #227 from clebergnu/crypto_cleanup

Crypto cleanup