qa/workunits/erasure-code/bench.sh:fix the error of getting encoding technique in...

qa/workunits/erasure-code/bench.sh:fix the error of getting encoding technique in ec test

 Isa / jerasure codec ‘technique’, obtained by the following statements,
 "eval technique_parameter=\$${plugin}2technique_${technique}",
 which generate a string such as "isa2technique_vandermonde",
 and assign the value of "technique_parameter" to "isa2technique_vandermonde".

 String such as "isa2technique_vandermonde" should have a preset value, but it does not,
 which will cause string "technique_parameter" to be empty.

 Run the script, prompt the following error message and exit:
 isa technique= is not a valid coding technique.  Choose one of the following: reed_sol_van,cauchy

 To fix the bug, specify a preset value for "technique_parameter":

 + isa2technique_vandermonde='reed_sol_van'
 + isa2technique_cauchy='cauchy'
 + jerasure2technique_vandermonde='reed_sol_van'
 + jerasure2technique_cauchy='cauchy_good'

 Signed-off-by: lijiaxu <lijiaxu@cmss.chinamobile.com>