Bluetooth: Mesh: Limit number of Bluetooth connections used by mesh

When a remote devices establishes a ble connection with the mesh device
with default identity and there are more available connection slots, the
mesh stack will try to restart connectable advertisements (either
pb-gatt or proxy depending on the provisioning state). This makes
difficult for an application to advertise own connectable advertisements
as the mesh stack will not let a chance the app to start advertising.

This change adds a Kconfig option that limits number of connection slots
that the mesh stack can use for own connectable advertisements.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>