TensorMechanicsPlasticTensile userobject added.

All tensile tests altered to use this instead of FiniteStrainTensile

Fixes #3877
FiniteStrainTensile Material removed.  gravity test modified to use TensorMechanicsPlasticTensile instead.  Fixes #3877
Merge pull request #3880 from WilkAndy/tensile_to_uo_3877

FiniteStrainTensile Material removed in favour of TensorMechanicsPlasticTensile