pycrypto is unmaintained for a while and requirements team advises using
pycryptodome or cryptography libraries instead (see openstack-dev thread
[0]).
troveclient uses pycrypto to generate random data/content in its tests
which does not require any specific cryptographic need. This commit
replaces it by os.urandom from Python standard library.
[0] http://lists.openstack.org/pipermail/openstack-dev/2017-March/113568.html
Change-Id: Ia43c5883a51e8412c4a238d127c13db39e863da1