Add a check for the total pgbouncer pool_size less than the PostgreSQL max_connections (#236)

[artemsafiyulin]

MR for issue #236
add 3 tasks:

• calculate pgbouncer total pool_size (if not defined pool_size, then calculate as pgbouncer_default_pool_size)
• get number of max connections from "postgresql_parameters" variable
• if total_pool_size > max_connections and pgbouncer_pools not empty, task will be failed and send special message

[vitabaks]

@artemsafiyulin Thanks!

calculate pgbouncer total pool_size (when pool_parameters not defined it will be 0)

In this case, the default_pool_size parameter has a value.

We have it declared as a variable pgbouncer_default_pool_size, it would not be bad to multiply this number by postgresql_databases for which pool_parameters are not declared. What do you think?

[artemsafiyulin]

@vitabaks I add support calculating with default_pool_size. Now we have 5 steps:

1. get count of databases in postgresql (without templates)
2. calculate size of pools defined in variable pgbouncer_pools
3. calculate total pool size (add to previous step count of not defined in pgbouncer_pools databases * default_pool_size)
4. get max_connections value
5. compare total_pool_size and max_connections