AmbigousTimeError on Timestamp.floor during dst change
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
pd.Timestamp('2018-11-04 01:55:17.869342-0500', tz='America/New_York').floor('T')
```
```
Traceback (most recent call last):
  File "/venv/lib/python3.5/site-packages/IPython/core/interactiveshell.py", line 3265, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-42bf9ce66d1d>", line 1, in <module>
    pd.Timestamp('2018-11-04 01:55:17.869342-0500', tz='America/New_York').floor('T')
  File "pandas/_libs/tslibs/timestamps.pyx", line 696, in pandas._libs.tslibs.timestamps.Timestamp.floor
  File "pandas/_libs/tslibs/timestamps.pyx", line 667, in pandas._libs.tslibs.timestamps.Timestamp._round
  File "pandas/_libs/tslibs/timestamps.pyx", line 903, in pandas._libs.tslibs.timestamps.Timestamp.tz_localize
  File "pandas/_libs/tslibs/conversion.pyx", line 963, in pandas._libs.tslibs.conversion.tz_localize_to_utc
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from '2018-11-04 01:55:00', try using the 'ambiguous' argument
```

#### Problem description

`AmbiguousTimeError` thrown on floor of timestamp that leads to time within dst change.

Similar issues happen on `ceil` and `round`:
```
pd.Timestamp('2018-11-04 01:55:17.869342-0500', tz='America/New_York').ceil('T')
pd.Timestamp('2018-11-04 01:55:17.869342-0500', tz='America/New_York').round('T')
```

#### Expected Output

```
Timestamp('2018-11-04 01:55:00-0500', tz='America/New_York')
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-38-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
pandas: 0.23.4
pytest: None
pip: 10.0.1
setuptools: 39.1.0
Cython: None
numpy: 1.15.2
scipy: None
pyarrow: None
xarray: None
IPython: 7.0.1
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
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>
