Introduced a new debugging extension gp_debug_numsegments to get / set
the default numsegments when creating tables.

gp_debug_get_create_table_default_numsegments() gets the default
numsegments.

gp_debug_set_create_table_default_numsegments(text) sets the default
numsegments in text format, valid values are:
- 'FULL': all the segments;
- 'RANDOM': pick a random set of segments each time;
- 'MINIMAL': the minimal set of segments;

gp_debug_set_create_table_default_numsegments(integer) sets the default
numsegments directly, valid range is [1, gp_num_contents_in_cluster].

gp_debug_reset_create_table_default_numsegments(text) or
gp_debug_reset_create_table_default_numsegments(integer) reset the
default numsegments to the specified value, and the value can be reused
later.

gp_debug_reset_create_table_default_numsegments() resets the default
numsegments to the value passed last time, if there is no previous call
to it the value is 'FULL'.

Refactored ICG test partial_table.sql to create partial tables with this
extension.