modem: get IMSI from ofono

There's no need to specify the IMSI manually in resource config
and it's also prone to errors. Let's take it from ofono.
Add a 'sim' feature to allow modem to auto-discover it,
otherwise if not supported leave that feature out of the config for that
modem and an imsi can still be manually providen.

Change-Id: I20f9e8d97775293925205e4ea576d814214bf1a8
diff --git a/example/resources.conf b/example/resources.conf
index fb2ac9f..cb730f5 100644
--- a/example/resources.conf
+++ b/example/resources.conf
@@ -69,32 +69,28 @@
 modem:
 - label: sierra_1st
   path: '/sys/devices/pci0000:00/0000:00:12.2/usb1/1-1/1-1.2'
-  imsi: '901700000009031'
   ki: '80A37E6FDEA931EAC92FFA5F671EFEAD'
   auth_algo: 'comp128v1'
   ciphers: [a5_0, a5_1]
-  features: ['sms', 'voice', 'ussd', 'gprs']
+  features: ['sms', 'voice', 'ussd', 'gprs', 'sim']
 
 - label: sierra_2nd
   path: '/sys/devices/pci0000:00/0000:00:12.2/usb1/1-1/1-1.3'
-  imsi: '901700000009029'
   ki: '00969E283349D354A8239E877F2E0866'
   auth_algo: 'comp128v1'
   ciphers: [a5_0, a5_1]
-  features: ['sms', 'voice', 'ussd', 'gprs']
+  features: ['sms', 'voice', 'ussd', 'gprs', 'sim']
 
 - label: ec20
   path: '/sys/devices/pci0000:00/0000:00:12.2/usb1/1-1/1-1.6'
-  imsi: '901700000009030'
   ki: 'BB70807226393CDBAC8DD3439FF54252'
   auth_algo: 'comp128v1'
   ciphers: [a5_0, a5_1]
-  features: ['sms', 'ussd', 'gprs']
+  features: ['sms', 'ussd', 'gprs', 'sim']
 
 - label: gobi2k
   path: '/sys/devices/pci0000:00/0000:00:12.2/usb1/1-1/1-1.5'
-  imsi: '901700000009032'
   ki: '2F70DCA43C45ACB97E947FDD0C7CA30A'
   auth_algo: 'comp128v1'
   ciphers: [a5_0, a5_1]
-  features: ['gprs']
+  features: ['gprs', 'sim']