use "#ifdef COLTE" macros for colte specific changes

[spencersevilla]

The following commits change the behavior of the EPC to do CoLTE-specific things that are not standard OAI behavior:

0c7e9a7 modifies the SPGW to consult the MySQL database and assign each UE a static IP address based on its IMSI. The previous OAI behavior was to dynamically assign IP addresses out of the pool. The most important functional change is that allocate_ue_ipv4_address now calls spgw_get_ip_from_imsi instead of pgw_get_free_ipv4_paa_address.

c0f32c4 modifies the HSS to also check the customers.enabled table value, instead of just consulting the users table.

In both cases, the extra code can stay, but I would like the actual logic to be wrapped in "#ifdef COLTE" macros, which are already triggered by ./scripts/build_component -C. This way, developers can compile with "-C" to get these features, otherwise they will get the standard OAI behavior they already expect.