T6364: CGNAT drop hard limit that allows only one translation rule

As PoC for CGNAT had a hard limit of using only one translation
rule for one internal pool.
Drop this limit and extend the usage number of the rules.

set nat cgnat rule 100 source pool 'int-01'
set nat cgnat rule 100 translation pool 'ext-01'
set nat cgnat rule 120 source pool 'vyos-int-02'
set nat cgnat rule 120 translation pool 'vyos-ext-02'