start_erl.data expected in non-existing location which requires to specify RELEASE_MUTABLE_DIR location

[velimir]

Steps to reproduce

1. Build new release with RELEASE_READ_ONLY
2. Deploy on read only file system.
3. Start asforeground.
4. See error: mkdir: cannot create directory ‘<YOUR_DIR_HERE>/var’: Read-only file system

Description of issue

#670 introduced a change, that modified expected location of start_erl.data.
Once release is built directory has the following structure:

├── bin
├── erts-9.3.3.5
├── lib
└── releases
    ├── 0.1.0
    ├── RELEASES
    └── start_erl.data

After the #670 change, start_erl.data file is expected at var/start_erl.data:

4ebfe9a#diff-bb0b28dac91fa3fff22ffea720be6334R10

distillery/priv/templates/release_rc_exec.eex

Lines 10 to 11 in b7805d2

	RELEASE_MUTABLE_DIR="${RELEASE_MUTABLE_DIR:-"${RELEASE_ROOT_DIR}/var"}"
	START_ERL_DATA="${RELEASE_MUTABLE_DIR}/start_erl.data"

[velimir]

closes as duplicate of #693