to_datetime function not working with %Y.%m.%d %H format
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import datetime as dt
print(dt.datetime.strptime('2012.01.01 1', '%Y.%m.%d %H')) 
# -> 2012-01-01 01:00:00
print(pd.to_datetime('2012.01.01 1', format='%Y.%m.%d %H')) 
# -> ValueError: time data '2012.01.01 1' doesn't match format specified

```
#### Problem description

When using `to_datetime` to parse a date that only includes an hour component, but not minutes and seconds, with a format that is otherwise similar to ISO8601 (such as the format `'%Y.%m.%d %H'`), a `ValueError` is raised (see above). This behavior is unexpected as `strptime` can parse the same date without any problem, using the same format string (see above).

I suspect that the problem is in the [`_format_is_iso` function of `pandas._libs.tslibs.parsing`](https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/parsing.pyx#L561), where it is just checked if the ISO format starts with the format given - so this format is recognized as being ISO-like. In this case, the format passed to `to_datetime` is ignored and `tslib.array_to_datetime` function is used to parse the date instead, which doesn't seem to be able to handle this kind of format.

My current workaround is to modify the dates to also have a minutes component (append `':00'` to every string) so that they can be parsed.

#### Expected Output

`2012-01-01 01:00:00` (same as when using `strptime`)

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8
LOCALE: de_DE.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 38.5.2
Cython: None
numpy: 1.14.2
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.2
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.0
openpyxl: 2.5.0
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
