config: Create ast_variable_find_in_list()

config: Create ast_variable_find_in_list()

Add
const char *ast_variable_find_in_list(const struct ast_variable *list,
   const char *variable);

ast_variable_find() requires a config category to search whereas
ast_variable_find_in_list() just needs the root list element which is
useful if you don't have a category.

Tested-by: George Joseph
Review: https://reviewboard.asterisk.org/r/4217/
........

Merged revisions 428733 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@428734 65c4cc65-6c06-0410-ace0-fbb531ad65f3