ap: fix SSID name length problem

Previously, SSIDs were used as the bridge interace names which caused
dhclient to fail if SSID was longer than 15 characters (max allowed
network interface name length is 15 chars).

Since a8fa9c9e6fc5ee4472cdfd8ffb1b902510844282, dhclient has been
considered as a critical child service and therefore its failure causes
puavo-wlanap to fail as well.

Before a8fa9c9e6fc5ee4472cdfd8ffb1b902510844282, dhclient failures did
not cause puavo-wlanap to fail.

This commit fixes the problem by defining that the interface name is
truncated md5 sum of the SSID.

PROS:
 - support for standard 32 char long SSIDs

 - interface names are guaranteed to be at most 15 chars long

CONS:
 - interface names are not human-friendly anymore (library script
   get-bridge can be used to mangle SSID to IFACE).

 - minor chance for getting interface name collision for two different
   SSIDs
Update VERSION to 0.7.3