Fix and simplify arbitrary_string. lp#1085048

The previous behaviour:

	In [4]: data_utils.arbitrary_string(size=3, base_text='test')
	Out[4]: 'tes'

	In [5]: data_utils.arbitrary_string(size=4, base_text='test')
	Out[5]: 'testtesttesttest'

didn't seem sane. Fixed to more sane:
	In [5]: data_utils.arbitrary_string(size=2, base_text='test')
	Out[5]: 'te'

	In [6]: data_utils.arbitrary_string(size=4, base_text='test')
	Out[6]: 'test'

	In [7]: data_utils.arbitrary_string(size=6, base_text='test')
	Out[7]: 'testte

Change-Id: Idf38329b26846b15de929e7d00c6dd1353bde068
1 file changed